Grandson of the eight-handed Starkader and the beautiful Alfhilde, called bær-serce (bare of mail) because he went into battle unharnessed. Hence, any man with the fighting fever on him.
“You say that I am berserker. And ... baresark I go to-morrow to the war.” —Rev. C. Kingsley; Hereward the Wake.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894