There is disagreement about whether "berserk" referred to "bare shirt", going shirtless, or "bear shirt", wearing a bear skin into battle, in the same way the ulfsarks wore wolf coats. These warriors would enter a rage in combat, where it is said their skin turned aside weapons. Some modern scholars suggest that the berserker rage may have been induced by consuming hallucinogenic mushrooms.

"A third I know, if great is my need Of fetters to hold my foe; Blunt do I make mine enemy's blade, Nor bites his sword or staff."

-- The Poetic Edda