Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. One who fights as if frenzied, like a Berserker.

Source : WordNet®

berserk
     adj : in a murderous frenzy as if possessed by a demon; "the
           soldier was completely amuck"; "berserk with grief"; "a
           berserk worker smashing windows" [syn: {amuck}, {amok},
            {demoniac}, {demoniacal}, {possessed(p)}]

berserk
     n : 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}]