The Collaborative International Dictionary of English v.0.48:

Severe \Se*vere"\, a. [Compar. Severer; superl. Severest.]
   [L. severus; perhaps akin to Gr. ??? awe, ??? revered, holy,
   solemn, Goth. swikns innocent, chaste: cf. F. s['e]v[`e]re.
   Cf. Asseverate, Persevere.]
   1. Serious in feeling or manner; sedate; grave; austere; not
      light, lively, or cheerful.
      [1913 Webster]

            Your looks alter, as your subject does,
            From kind to fierce, from wanton to severe.
                                                  --Waller.
      [1913 Webster]

   2. Very strict in judgment, discipline, or government; harsh;
      not mild or indulgent; rigorous; as, severe criticism;
      severe punishment. "Custody severe." --Milton.
      [1913 Webster]

            Come! you are too severe a moraler.   --Shak.
      [1913 Webster]

            Let your zeal, if it must be expressed in anger, be
            always more severe against thyself than against
            others.                               --Jer. Taylor.
      [1913 Webster]

   3. Rigidly methodical, or adherent to rule or principle;
      exactly conformed to a standard; not allowing or employing
      unneccessary ornament, amplification, etc.; strict; --
      said of style, argument, etc. "Restrained by reason and
      severe principles." --Jer. Taylor.
      [1913 Webster]

            The Latin, a most severe and compendious language.
                                                  --Dryden.
      [1913 Webster]

   4. Sharp; afflictive; distressing; violent; extreme; as,
      severe pain, anguish, fortune; severe cold.
      [1913 Webster]

   5. Difficult to be endured; exact; critical; rigorous; as, a
      severe test.
      [1913 Webster]

   Syn: Strict; grave; austere; stern; morose; rigid; exact;
        rigorous; hard; rough; harsh; censorious; tart;
        acrimonious; sarcastic; satirical; cutting; biting;
        keen; bitter; cruel. See Strict.
        [1913 Webster] -- Se*vere"ly, adv. -- Se*vere"ness,
        n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

severe
    adj 1: intensely or extremely bad or unpleasant in degree or
           quality; "severe pain"; "a severe case of flu"; "a
           terrible cough"; "under wicked fire from the enemy's
           guns"; "a wicked cough" [syn: severe, terrible,
           wicked]
    2: very strong or vigorous; "strong winds"; "a hard left to the
       chin"; "a knockout punch"; "a severe blow" [syn: hard,
       knockout, severe]
    3: severely simple; "a stark interior" [syn: austere,
       severe, stark, stern]
    4: unsparing and uncompromising in discipline or judgment; "a
       parent severe to the pitch of hostility"- H.G.Wells; "a hefty
       six-footer with a rather severe mien"; "a strict
       disciplinarian"; "a Spartan upbringing" [syn: severe,
       spartan]
    5: causing fear or anxiety by threatening great harm; "a
       dangerous operation"; "a grave situation"; "a grave illness";
       "grievous bodily harm"; "a serious wound"; "a serious turn of
       events"; "a severe case of pneumonia"; "a life-threatening
       disease" [syn: dangerous, grave, grievous, serious,
       severe, life-threatening]
    6: very bad in degree or extent; "a severe worldwide
       depression"; "the house suffered severe damage"

Moby Thesaurus II by Grady Ward, 1.0:

384 Moby Thesaurus words for "severe":
   Draconian, Herculean, Siberian, Spartan, Spartanic, abrupt,
   absolute, absolutist, absolutistic, abstruse, acerbic, acid,
   acidulous, acrid, acrimonious, acute, afflictive, aggressive,
   agonizing, algid, aloof, arbitrary, arctic, arduous, aristocratic,
   arrogant, ascetic, asperous, astringent, atrocious, austere,
   authoritarian, authoritative, autocratic, awful, badly, bald, bare,
   basic, bearish, beastly, below zero, biting, bitter, bitterly cold,
   bleak, blistering, bluff, blunt, blustering, blustery, boreal,
   bossy, brash, brisk, brumal, brusque, brutal, burdensome, candid,
   caustic, cavalier, chaste, churlish, close, cold, cold as charity,
   cold as death, cold as ice, cold as marble, common, commonplace,
   complex, constant, cramping, crimpy, crisp, critical, crude, cruel,
   crusty, curt, cutting, dangerous, delicate, demanding, despotic,
   dictatorial, difficile, difficult, dire, direct, disagreeable,
   disciplined, distressing, domineering, double-edged, dour, drastic,
   dreadful, dry, dull, edged, effortful, elementary, escharotic,
   essential, even, exact, exacting, excessive, excruciating, exigent,
   exorbitant, express, extravagant, extreme, faithful, fastidious,
   fatal, feudal, fierce, fine, flinty, forbidding, formidable, frank,
   freezing, freezing cold, frigid, fundamental, furious, gelid,
   glacial, glowering, gnawing, grave, gravely, great, grievous, grim,
   grinding, griping, gruff, hairy, hard, hard-earned, hard-fought,
   hardly, harrowing, harsh, harshly, heavy, heavy-handed, hibernal,
   hiemal, high-handed, homely, homespun, homogeneous, hostile,
   hurtful, hurting, hyperborean, ice-cold, ice-encrusted, icelike,
   icy, immoderate, imperative, imperial, imperious, incisive,
   inclement, indivisible, inerrable, inerrant, inexorable,
   infallible, inflexible, inhospitable, inhuman, inornate,
   intemperate, intense, intensely, intricate, iron-willed,
   irreducible, irritating, jawbreaking, keen, knotted, knotty,
   laborious, lean, lordly, magisterial, magistral, markedly,
   masterful, mathematical, matter-of-fact, mean, merciless, mere,
   meticulous, micrometrically precise, microscopic, modest, monastic,
   monocratic, monolithic, mordacious, mordant, mortal, mortified,
   natural, neat, nice, nipping, nippy, no picnic, nose-tickling,
   not easy, numbing, obdurate, of a piece, onerous, open, operose,
   oppressive, outrageous, overbearing, overruling, painful,
   painfully, painstaking, paroxysmal, penetrating, peremptory,
   piercing, pinching, pinpoint, piquant, pitiless, plain,
   plain-speaking, plain-spoken, poignant, precise, primal, primary,
   primitive, prosaic, prosing, prosy, pungent, punishing, punitive,
   pure, pure and simple, racking, raw, refined, relentless,
   religious, religiously exact, repressive, restrictive, rigid,
   rigorous, rigorously, rough, roughly, rugged, rustic, ruthless,
   savage, scathing, scientific, scientifically exact,
   self-disciplined, serious, set with thorns, severe, severely,
   sharp, shooting, short, simon-pure, simple, simple-speaking,
   single, sleety, slushy, smart, snappy, snippy, sober, sore, sour,
   spare, sparse, spasmatic, spasmic, spasmodic, spiny, splitting,
   square, stabbing, stark, steep, stern, stiff, stinging, stone-cold,
   stony, stormy, straightforward, strait-laced, strenuous, strict,
   strident, stringent, subtle, subzero, supercooled, suppressive,
   surly, tart, taxing, terminal, thorny, ticklish, toilsome,
   tormenting, torturous, tough, trenchant, tricky, truculent,
   turbulent, tyrannical, tyrannous, unadorned, unaffected, unbending,
   uncluttered, uncompromising, unconscionable, undecorated,
   undeviating, undifferenced, undifferentiated, unelaborate,
   unembellished, unerring, unfancy, unfeeling, unfussy, ungentle,
   uniform, unimaginative, unornate, unpleasant, unpoetical,
   unsmiling, unsparing, unsympathetic, unvarnished, unyielding,
   uphill, vehement, venomous, violent, virulent, vitriolic, weighty,
   wicked, winterbound, winterlike, wintery, wintry