berserker noun orberserk\“\ (-s) Etymology: Old Norse berserkr, from ber- (stem of björn bear) + serkr shirt — more at bear, sark 1.: an ancient Scandinavian warrior reputed to be invulnerable, of enormous strength, and filled with wild frenzy in battle 2.: one whose actions are marked by a headstrong intractable spirit or by reckless defiance (as of orthodox views or attitudes) < a political berserker whose course was completely unpredictable >