The Collaborative International Dictionary of English v.0.48:

Fierce \Fierce\, a. [Compar. Fiercer; superl. Fiercest.]
   [OE. fers, fiers, OF. fier, nom. fiers, fierce, savage,
   cruel, F. fier proud, from L. ferus wild, savage, cruel;
   perh. akin to E. bear the animal. Cf. Feral, Ferocity.]
   1. Furious; violent; unrestrained; impetuous; as, a fierce
      wind.
      [1913 Webster]

            His fierce thunder drove us to the deep. --Milton.
      [1913 Webster]

   2. Vehement in anger or cruelty; ready or eager to kill or
      injure; of a nature to inspire terror; ferocious. "A
      fierce whisper." --Dickens. "A fierce tyrant." --Pope.
      [1913 Webster]

            The fierce foe hung upon our broken rear. --Milton.
      [1913 Webster]

            Thou huntest me as a fierce lion.     --Job. x. 16.
      [1913 Webster]

   3. Excessively earnest, eager, or ardent.

   Syn: Ferocious; savage; cruel; vehement; impetuous;
        barbarous; fell. See Ferocious. -- Fierce"ly, adv.
        -- Fierce"ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

fierce
    adj 1: marked by extreme and violent energy; "a ferocious
           beating"; "fierce fighting"; "a furious battle" [syn:
           ferocious, fierce, furious, savage]
    2: marked by extreme intensity of emotions or convictions;
       inclined to react violently; fervid; "fierce loyalty"; "in a
       tearing rage"; "vehement dislike"; "violent passions" [syn:
       fierce, tearing, vehement, violent, trigger-happy]
    3: ruthless in competition; "cutthroat competition"; "bowelless
       readiness to take advantage" [syn: cutthroat, fierce,
       bowelless]
    4: violently agitated and turbulent; "boisterous winds and
       waves"; "the fierce thunders roar me their music"- Ezra
       Pound; "rough weather"; "rough seas" [syn: boisterous,
       fierce, rough]

Moby Thesaurus II by Grady Ward, 1.0:

227 Moby Thesaurus words for "fierce":
   Draconian, Tartarean, abandoned, acid, acidulous, acrid,
   acrimonious, acute, aggressive, amok, angry, animal, antagonistic,
   anthropophagous, astringent, atrocious, awful, barbaric, barbarous,
   battling, beastly, bellicose, belligerent, bellowing, berserk,
   bestial, biting, bitter, bloodthirsty, bloody, bloody-minded,
   brutal, brutalized, brute, brutish, cannibalistic, carried away,
   caustic, chauvinist, chauvinistic, combative, concentrated,
   contentious, cruel, cruel-hearted, cutting, dangerous, delirious,
   demoniac, demoniacal, desperate, devilish, diabolic, dire,
   distracted, double-edged, drastic, dreadful, ecstatic, edged,
   enemy, enraged, enraptured, escharotic, excessive, exorbitant,
   exquisite, extravagant, extreme, fell, feral, ferine, ferocious,
   fiendish, fiendlike, fiery, fighting, fighting mad, frantic,
   frenzied, full of fight, fulminating, fuming, furious, great, grim,
   haggard, harsh, hawkish, hellish, hog-wild, homicidal, hopping mad,
   hostile, hotheaded, howling, hysterical, immoderate, impetuous,
   in a rage, in a transport, in hysterics, incisive, infernal,
   infuriate, infuriated, inhuman, inhumane, inimical, inordinate,
   intemperate, intense, intoxicated, intractable, jingo, jingoish,
   jingoist, jingoistic, keen, kill-crazy, mad, maddened, madding,
   malign, malignant, maniac, martial, menacing, merciless, militant,
   militaristic, military, mordacious, mordant, murderous,
   noncivilized, offensive, orgasmic, orgiastic, outrageous,
   passionate, penetrating, piercing, pitiless, poignant, possessed,
   pugnacious, quarrelsome, rabid, racking, raging, ramping, ranting,
   raving, raving mad, ravished, rigorous, roaring, roaring mad,
   rough, running mad, ruthless, saber-rattling, sadistic, sanguinary,
   sanguineous, satanic, savage, scathing, scrappy, severe, sharkish,
   sharp, simmering, slavering, soldierlike, soldierly, splitting,
   stabbing, stinging, storming, stormy, strident, stringent,
   subhuman, superlative, supreme, tameless, tart, tempestuous,
   terrible, threatening, tigerish, tornadic, tough, transcendent,
   transported, trenchant, trigger-happy, truculent, tumultuous,
   turbulent, unchristian, uncivilized, unconscionable,
   uncontrollable, unfriendly, ungentle, unhuman, unpacific,
   unpeaceable, unpeaceful, untamed, vehement, venomous, vicious,
   violent, virulent, vitriolic, volcanic, warlike, warmongering,
   warring, wild, wild-eyed, wild-looking, wolfish