The Collaborative International Dictionary of English v.0.48:

Stiff \Stiff\, a. [Compar. Stiffer; superl. Stiffest.] [OE.
   stif, AS. st[imac]f; akin to D. stijf, G. steif, Dan. stiv,
   Sw. styf, Icel. st[imac]fr, Lith. stipti to be stiff; cf. L.
   stipes a post, trunk of a tree, stipare to press, compress.
   Cf. Costive, Stifle, Stipulate, Stive to stuff.]
   1. Not easily bent; not flexible or pliant; not limber or
      flaccid; rigid; firm; as, stiff wood, paper, joints.
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            [They] rising on stiff pennons, tower
            The mid aerial sky.                   --Milton.
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   2. Not liquid or fluid; thick and tenacious; inspissated;
      neither soft nor hard; as, the paste is stiff.
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   3. Firm; strong; violent; difficult to oppose; as, a stiff
      gale or breeze.
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   4. Not easily subdued; unyielding; stubborn; obstinate;
      pertinacious; as, a stiff adversary.
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            It is a shame to stand stiff in a foolish argument.
                                                  --Jer. Taylor.
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            A war ensues: the Cretans own their cause,
            Stiff to defend their hospitable laws. --Dryden.
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   5. Not natural and easy; formal; constrained; affected;
      starched; as, stiff behavior; a stiff style.
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            The French are open, familiar, and talkative; the
            Italians stiff, ceremonious, and reserved.
                                                  --Addison.
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   6. Harsh; disagreeable; severe; hard to bear. [Obs. or
      Colloq.] "This is stiff news." --Shak.
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   7. (Naut.) Bearing a press of canvas without careening much;
      as, a stiff vessel; -- opposed to crank. --Totten.
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   8. Very large, strong, or costly; powerful; as, a stiff
      charge; a stiff price. [Slang]
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   Stiff neck, a condition of the neck such that the head can
      not be moved without difficulty and pain.
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   Syn: Rigid; inflexible; strong; hardly; stubborn; obstinate;
        pertinacious; harsh; formal; constrained; affected;
        starched; rigorous.
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WordNet (r) 3.0 (2006):

stiff
    adv 1: extremely; "bored stiff"; "frightened stiff"
    2: in a stiff manner; "his hands lay stiffly" [syn: stiffly,
       stiff]
    adj 1: not moving or operating freely; "a stiff hinge"
    2: powerful; "a stiff current"; "a stiff breeze"
    3: rigidly formal; "a starchy manner"; "the letter was stiff and
       formal"; "his prose has a buckram quality" [syn: starchy,
       stiff, buckram]
    4: having a strong physiological or chemical effect; "a potent
       toxin"; "potent liquor"; "a potent cup of tea", "a stiff
       drink" [syn: potent, strong, stiff] [ant: impotent]
    5: marked by firm determination or resolution; not shakable;
       "firm convictions"; "a firm mouth"; "steadfast resolve"; "a
       man of unbendable perseverence"; "unwavering loyalty" [syn:
       firm, steadfast, steady, stiff, unbendable,
       unfaltering, unshakable, unwavering]
    6: incapable of or resistant to bending; "a rigid strip of
       metal"; "a table made of rigid plastic"; "a palace guardsman
       stiff as a poker"; "stiff hair"; "a stiff neck" [syn:
       rigid, stiff]
    7: very drunk [syn: besotted, blind drunk, blotto,
       crocked, cockeyed, fuddled, loaded, pie-eyed,
       pissed, pixilated, plastered, slopped, sloshed,
       smashed, soaked, soused, sozzled, squiffy, stiff,
       tight, wet]
    n 1: an ordinary man; "a lucky stiff"; "a working stiff"
    2: the dead body of a human being; "the cadaver was intended for
       dissection"; "the end of the police search was the discovery
       of a corpse"; "the murderer confessed that he threw the stiff
       in the river"; "honor comes to bless the turf that wraps
       their clay" [syn: cadaver, corpse, stiff, clay,
       remains]

Moby Thesaurus II by Grady Ward, 1.0:

522 Moby Thesaurus words for "stiff":
   Arab, Bowery bum, Latinate, a bit much, abandoned, abusive,
   adamant, adamantine, afflictive, alcoholic, all thumbs, aloof,
   arduous, arid, artificial, ashes, austere, awkward, balker,
   balky horse, barren, beach bum, beachcomber, beggar,
   beggarly fellow, blah, blank, blighter, blind, blind drunk,
   bloodless, blotto, blue-collar worker, blunderheaded, blundering,
   bo, body, bold, bombastic, bones, boorish, boundless, breadwinner,
   brisk, brittle, buckram, budmash, bum, bumbling, bummer, bungling,
   butterfingered, cadaver, caitiff, carcass, careless, carrion,
   cartilaginous, cast-iron, casual, casual laborer, ceremonious,
   challenging, characterless, cheapskate, chewy, chilly, clay,
   clownish, clumsy, clumsy-fisted, cohesive, cold, colorless,
   common laborer, compact, cool, coriaceous, corpse, corpus delicti,
   costly, cramped, crock, crowbait, cruel, cumbersome, cumbrous,
   day laborer, dead, dead as mutton, dead body, dead man,
   dead person, dear, dear-bought, decedent, dense, derelict,
   determined, devil, difficult, dismal, distressing, dog, dogged,
   dogie, dour, draggy, drastic, drearisome, dreary, drifter,
   drunkard, dry, dry bones, dryasdust, dull, durable, dust, dusty,
   earth, effete, egregious, elephantine, embalmed corpse, employee,
   empty, energetic, enormous, etiolated, exaggerated, excessive,
   excruciating, exhausting, exorbitant, expensive, extravagant,
   extreme, fabulous, factory worker, fade, fancy, fatiguing, fibrous,
   fingers all thumbs, firm, fit for sea, flat, flinty, flunky,
   food for worms, forced, forceful, formal, free lance, free-lancer,
   fresh, full-time worker, fumbling, fundamentalist, gamin, gamine,
   garron, gauche, gawkish, gawky, gigantic, gluttonous, goat,
   gone to seed, good-for-naught, good-for-nothing, graceless, grim,
   gristly, guinde, gusty, guttersnipe, hack, had, halting,
   ham-fisted, ham-handed, hand, hard, hard-core, hardened, hardy,
   harrowing, harsh, haughty, heavy, heavy-handed, helpless,
   hidebound, high, high-priced, ho-hum, hobo, hollow, homeless waif,
   howling, hulking, hulky, human wreck, hurtful, hyperbolic,
   hypertrophied, idler, immoderate, immovable, immutable, implacable,
   impliable, in buckram, inane, incontinent, indomitable,
   industrial worker, inelastic, inelegant, inexcitable, inexorable,
   inflexible, inkhorn, inordinate, insipid, intemperate, intense,
   intransigent, iron, ironbound, ironclad, ironhanded,
   irreconcilable, jade, jejune, jobber, jobholder, jughead, labored,
   laborer, laboring man, laborious, landloper, lasting,
   late lamented, lazzarone, leaden, leatherlike, leathery, left-hand,
   left-handed, lifeless, loafer, losel, loutish, low-spirited,
   lowlife, lubberly, lumbering, lumpish, luxurious, maladroit,
   mannered, mauvais sujet, mean wretch, menial, merciless, migrant,
   miser, moiler, monstrous, mortal remains, mucker, mudlark,
   mummification, mummy, muscle-bound, nag, navvy, no-good,
   not affordable, oafish, obdurate, obstinate, of great cost,
   office temporary, on the downgrade, organic remains, orthodox, out,
   out cold, out of bounds, out of condition, out of shape,
   out of sight, out of training, outrageous, overbig, overcome,
   overdeveloped, overgreat, overgrown, overlarge, overmuch,
   overpowering, overweening, overwhelming, painful, pale, pallid,
   paralyzed, passed out, pauvre diable, pedantic, pedestrian,
   petrified, piker, pilgarlic, plodding, plug, pointless, pokerlike,
   poky, pompous, ponderous, poor creature, poor devil, potent,
   powerful, premium, pricey, prim, procrustean, proletarian,
   punishing, punitive, purist, puristic, puritan, puritanic,
   ragamuffin, ragman, ragpicker, ramrodlike, relentless, relics,
   reliquiae, remains, renitent, reserved, resistant, resolute,
   resolved, rich, rigid, rigorist, rigoristic, rigorous, roarer,
   rock-ribbed, rockbound, rodlike, rogue, ropy, rosinante, rough,
   rounder, roustabout, run to seed, rusty, sad case, sad sack,
   salaried worker, scalawag, sea-kindly, seaworthy,
   self-employed person, servant, sesquipedalian, severe, sinewy,
   skeleton, ski bum, skid-row bum, skinflint, slipping, sloppy, slow,
   snobbish, snooty, snug, soft, solemn, solid, solidified, spanking,
   spiritless, standoffish, starch, starched, starchy, stark, staunch,
   steady, steely, steep, sterile, stern, stiff as buckram, stiffened,
   stilted, stodgy, stone-cold, stone-dead, stoned, straightlaced,
   straitlaced, stray, street Arab, street urchin, stringy, strong,
   stubborn, stuffy, sumptuous, sundowner, superficial, surf bum,
   swagman, swagsman, tasteless, tatterdemalion, taut, tedious,
   temporary, tenacious, tender, tenement of clay, tennis bum, tense,
   the dead, the deceased, the defunct, the departed, the loved one,
   thick, tight, tightwad, tiring, toiler, toilsome, too much, top,
   tormenting, torturous, tough, tough as leather, tramp, truant,
   turgid, turnpiker, unaffected, unalterable, unbearable, unbending,
   unbridled, unchangeable, uncompromising, unconscionable, uncouth,
   under the table, undue, unflexible, unfriendly, ungainly, ungiving,
   ungraceful, unhandy, unlively, unmoved, unpayable, unreasonable,
   unrelaxed, unrelenting, unrestrained, untiring, unwieldy,
   unyielding, uphill, uptight, urchin, vag, vagabond, vagrant, vapid,
   vaurien, vigorous, virgate, viscid, wage earner, wage slave,
   wageworker, waif, waifs and strays, wastrel, waterproof,
   watertight, weatherly, whistler, wiry, wooden, worker, workgirl,
   workhand, working girl, workingman, workingwoman, workman,
   worthless fellow, wretch