The Collaborative International Dictionary of English v.0.48:

Spare \Spare\, v. t. [imp. & p. p. Spared; p. pr. & vb. n.
   Sparing.] [AS. sparian, fr. spaer spare, sparing, saving;
   akin to D. & G. sparen, OHG. spar?n, Icel. & Sw. spara, Dan.
   spare See Spare, a.]
   1. To use frugally or stintingly, as that which is scarce or
      valuable; to retain or keep unused; to save. "No cost
      would he spare." --Chaucer.
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            [Thou] thy Father's dreadful thunder didst not
            spare.                                --Milton.
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            He that hath knowledge, spareth his words. --Prov.
                                                  xvii. 27.
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   2. To keep to one's self; to forbear to impart or give.
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            Be pleased your plitics to spare.     --Dryden.
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            Spare my sight the pain
            Of seeing what a world of tears it costs you.
                                                  --Dryden.
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   3. To preserve from danger or punishment; to forbear to
      punish, injure, or harm; to show mercy to.
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            Spare us, good Lord.                  --Book of
                                                  Common Prayer.
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            Dim sadness did not spare
            That time celestial visages.          --Milton.
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            Man alone can whom he conquers spare. --Waller.
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   4. To save or gain, as by frugality; to reserve, as from some
      occupation, use, or duty.
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            All the time he could spare from the necessary cares
            of his weighty charge, he ?estowed on . . . serving
            of God.                               --Knolles.
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   5. To deprive one's self of, as by being frugal; to do
      without; to dispense with; to give up; to part with.
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            Where angry Jove did never spare
            One breath of kind and temperate air. --Roscommon.
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            I could have better spared a better man. --Shak.
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   To spare one's self.
      (a) To act with reserve. [Obs.]
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                Her thought that a lady should her spare.
                                                  --Chaucer.
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      (b) To save one's self labor, punishment, or blame.
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The Collaborative International Dictionary of English v.0.48:

Spare \Spare\, v. i.
   1. To be frugal; not to be profuse; to live frugally; to be
      parsimonious.
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            I, who at some times spend, at others spare,
            Divided between carelessness and care. --Pope.
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   2. To refrain from inflicting harm; to use mercy or
      forbearance.
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            He will not spare in the day of vengeance. --Prov.
                                                  vi. 34.
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   3. To desist; to stop; to refrain. [Obs.] --Chaucer.
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The Collaborative International Dictionary of English v.0.48:

Spare \Spare\, a. [Compar. Sparer; superl. Sparest; -- not
   used in all the senses of the word.] [AS. spaer sparing. Cf.
   Spare, v. t. ]
   1. Scanty; not abundant or plentiful; as, a spare diet.
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   2. Sparing; frugal; parsimonious; chary.
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            He was spare, but discreet of speech. --Carew.
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   3. Being over and above what is necessary, or what must be
      used or reserved; not wanted, or not used; superfluous;
      as, I have no spare time.
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            If that no spare clothes he had to give. --Spenser.
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   4. Held in reserve, to be used in an emergency; as, a spare
      anchor; a spare bed or room.
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   5. Lean; wanting flesh; meager; thin; gaunt.
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            O, give me the spare men, and spare me the great
            ones.                                 --Shak.
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   6. Slow. [Obs. or prov. Eng.] --Grose.
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The Collaborative International Dictionary of English v.0.48:

Spare \Spare\, n.
   1. The act of sparing; moderation; restraint. [Obs.]
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            Killing for sacrifice, without any spare. --Holland.
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   2. Parsimony; frugal use. [Obs.] --Bacon.
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            Poured out their plenty without spite or spare.
                                                  --Spenser.
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   3. An opening in a petticoat or gown; a placket. [Obs.]
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   4. That which has not been used or expended.
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   5. (Tenpins) The right of bowling again at a full set of
      pins, after having knocked all the pins down in less than
      three bowls. If all the pins are knocked down in one bowl
      it is a double spare; in two bowls, a single spare. For
      the meaning in modern bowling, see sense 6.
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   6. (Bowling) The act of knocking down all ten pins in two
      bowls, which entitles the bowler to add the number of pins
      knocked down in the next bowl to the score for the frame
      in which the spare occurred.
      [PJC]

WordNet (r) 3.0 (2006):

spare
    adj 1: thin and fit; "the spare figure of a marathon runner"; "a
           body kept trim by exercise" [syn: spare, trim]
    2: more than is needed, desired, or required; "trying to lose
       excess weight"; "found some extra change lying on the
       dresser"; "yet another book on heraldry might be thought
       redundant"; "skills made redundant by technological advance";
       "sleeping in the spare room"; "supernumerary ornamentation";
       "it was supererogatory of her to gloat"; "delete superfluous
       (or unnecessary) words"; "extra ribs as well as other
       supernumerary internal parts"; "surplus cheese distributed to
       the needy" [syn: excess, extra, redundant, spare,
       supererogatory, superfluous, supernumerary, surplus]
    3: not taken up by scheduled activities; "a free hour between
       classes"; "spare time on my hands" [syn: spare, free]
    4: kept in reserve especially for emergency use; "a reserve
       supply of food"; "a spare tire"; "spare parts"
    5: lacking in amplitude or quantity; "a bare livelihood"; "a
       scanty harvest"; "a spare diet" [syn: bare(a), scanty,
       spare]
    6: lacking embellishment or ornamentation; "a plain hair style";
       "unembellished white walls"; "functional architecture
       featuring stark unornamented concrete" [syn: plain, bare,
       spare, unembellished, unornamented]
    n 1: an extra component of a machine or other apparatus [syn:
         spare part, spare]
    2: an extra car wheel and tire for a four-wheel vehicle [syn:
       fifth wheel, spare]
    3: a score in tenpins; knocking down all ten after rolling two
       balls
    v 1: refrain from harming [syn: spare, save]
    2: save or relieve from an experience or action; "I'll spare you
       from having to apologize formally"
    3: give up what is not strictly needed; "he asked if they could
       spare one of their horses to speed his journey" [syn:
       spare, give up, part with, dispense with]
    4: use frugally or carefully

Moby Thesaurus II by Grady Ward, 1.0:

481 Moby Thesaurus words for "spare":
   Lenten, Scotch, Spartan, abandon, abjure, absolve, abstain,
   abstain from, abstemious, accessory, additional, afford, agent,
   allow, alternate, alternative, amnesty, analogy, ancillary,
   angular, another, as a bonus, ascetic, austere, auxiliary, avoid,
   award, backup, balance, bald, bare, basic, bear, bestow, bonus,
   bony, cadaverous, candid, canny, careful, casual, cede, change,
   changeling, chary, chaste, cheeseparing, collateral, common,
   commonplace, comparison, conserve, conserved, conserving,
   contributory, copy, copyright, counterfeit, de trop, deliver,
   deputy, direct, discharge, disgorge, dispensable, dispense,
   dispense from, dispense with, dispose of, dividend, do without,
   donate, double, drop, dry, dull, dummy, dump, dwarfed, dwarfish,
   economic, economizing, elementary, endure, equal, equivalent,
   ersatz, eschew, essential, except, excess, exchange, exculpate,
   excuse, exempt, exiguous, exonerate, expendable, expletive, extra,
   fake, farther, fill-in, flat, flat-chested, fleshless,
   for lagniappe, forbear, forehanded, forgive, forgo, forsake,
   forswear, frank, free, fresh, frugal, fundamental, further,
   gangling, gangly, gaunt, gawky, get along without, get rid of,
   ghost, ghostwriter, give, give absolution, give away,
   give dispensation from, give quarter, give up, grant amnesty to,
   grant forgiveness, grant immunity, grant remission, gratuitous,
   gratuity, guard, have done with, have mercy upon, have pity, held,
   held back, held in reserve, held out, hold, hold aloof from,
   hold back, hold off, homely, homespun, homogeneous, idle,
   imitation, impoverished, in abeyance, in addition, in excess,
   in hand, in reserve, incidental, indivisible, irreducible, jejune,
   keep, keep alive, keep back, keep from, keep in hand, keep intact,
   keep inviolate, keep safe, keep up, kept, kiss good-bye,
   labor-saving, lagniappe, lank, lanky, lay aside, lay away, lay by,
   lay in, lay up, lean, lean-fleshed, lean-looking, left, leftover,
   leisure, leisured, let alone, let go, let go by, let have, let off,
   let up on, liberate, limited, locum tenens, maintain,
   make a sacrifice, makeshift, margin, matter-of-fact, meager, mean,
   melt, mere, metaphor, metonymy, mint, miserly, mock, modest,
   money-saving, monolithic, more, more than enough, narrow, natural,
   neat, needless, net, never touch, new, next best thing, niggardly,
   nonessential, not destroy, not endanger, not expend, not touch,
   not use, not use up, not waste, occasional, odd, of a piece, off,
   open, original, other, out-of-the-way, outstanding, over,
   over and above, overage, overmeasure, overplus, overrun, overset,
   overstock, oversupply, paltry, pardon, parsimonious, part with,
   part-time, pass up, patent, penny-wise, personnel, phony, pinch,
   pinch hitter, plain, plain-speaking, plain-spoken, pleonastic,
   plus, poor, pourboire, preserve, preserved, primal, primary,
   pristine, prolix, prosaic, prosing, prosy, protect, provident,
   provisional, proxy, prudent, prudential, puny, pure,
   pure and simple, put aside, put by, quitclaim, rawboned, recant,
   redeem, redundant, refrain, refrain from, register, relax, release,
   relent, relief, relieve, relinquish, remainder, remaining,
   remanent, remise, remit, render up, renounce, replacement,
   representative, reprieve, rescue, reserve, reserved, reserves,
   resign, retained, retired, retract, ringer, rustic, sacrifice,
   salt away, save, save the necessity, saved, saving, scant, scanty,
   scraggy, scrawny, scrimp, scrimping, scrimpy, second string,
   secondary, semiretired, severe, short, shrive, shun, side, sign,
   simon-pure, simple, simple-speaking, single, skeletal, skimp,
   skimping, skimpy, skinny, slender, slight, slim, small, sober,
   something extra, spare the price, spares, sparing, sparse, spidery,
   spindling, spindly, stand, stand aloof from, stand-in, stark,
   starvation, stingy, stint, stinted, stopgap, stored,
   straightforward, straitened, stunted, sub, subsistence,
   substituent, substitute, substitution, succedaneum, supererogatory,
   superfluous, supernumerary, superseder, supplanter, supplemental,
   supplementary, support, surplus, surplusage, surrender, surrogate,
   surviving, suspended, sustain, swear off, symbol, synecdoche,
   take pity on, tautologic, tautologous, temporary, tentative, thaw,
   thin, thin-bellied, thin-fleshed, third string, thrifty, throw up,
   tide, time-saving, tip, to spare, token, twiggy, ulterior,
   unadorned, unaffected, unapplied, unbeaten, uncalled-for,
   uncluttered, unconsumed, undergo, undersized, understudy,
   underweight, undifferenced, undifferentiated, unemployed,
   unessential, unexercised, unexpended, unhandled, uniform,
   unimaginative, unnecessary, unneeded, unnourishing, unnutritious,
   unoccupied, unpoetical, unspent, untapped, untouched, untrodden,
   unused, unutilized, unvarnished, unwasteful, uphold, utility,
   utility player, vacate, verbose, vicar, vicarious, vice-president,
   vice-regent, waive, waived, watered, watery, well afford, wiry,
   withheld, withhold, yield