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]

WordNet (r) 3.0 (2006):

berserker
    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]