The Collaborative International Dictionary of English v.0.48:

Berserk \Ber"serk\, Berserker \Ber"serk*er\, n. [Icel.
   berserkr.]
   1. (Scand. Myth.) One of a class of legendary heroes, who
      fought frenzied by intoxicating liquors, and naked,
      regardless of wounds. --Longfellow.
      [1913 Webster]

   2. One who fights as if frenzied, like a Berserker.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Berserk \Ber*serk"\ (b[~e]r*s[~e]rk"; b[~e]r*z[~e]rk";
   b[~e]r"s[~e]rk) a. [See berserk, n.]
   frenzied; crazed; usually in predicate position.
   [PJC]

WordNet (r) 3.0 (2006):

berserk
    adj 1: frenzied as if possessed by a demon; "the soldier was
           completely amuck"; "berserk with grief"; "a berserk
           worker smashing windows" [syn: amuck, amok,
           berserk, demoniac, demoniacal, possessed(p)]
    n 1: one of the ancient Norse warriors legendary for working
         themselves into a frenzy before a battle and fighting with
         reckless savagery and insane fury [syn: berserker,
         berserk]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "berserk":
   Dionysiac, abandoned, amok, bacchic, bellowing, carried away,
   corybantic, crazed, delirious, demoniac, desperate, distracted,
   ecstatic, enraptured, feral, ferocious, fierce, frantic, frenetic,
   frenzied, fulminating, furious, haggard, hog-wild, howling,
   hysterical, in a transport, in hysterics, intoxicated,
   like one possessed, mad, madding, maenadic, maniac, maniacal,
   orgasmic, orgiastic, possessed, rabid, raging, ramping, ranting,
   raving, raving mad, ravished, roaring, running mad, running wild,
   stark-raving mad, storming, transported, uncontrollable, violent,
   wild, wild-eyed, wild-looking