[Norwegian berserk, Old Norse berserkr ("bearskin")]
In premedieval and medieval Norse history and folklore: a member of unruly warrior gangs dressed in bearskin. They were believed to be able to transform themselves into bears (Old Norse ber means bear).
Their rage and wild behaviour are sometimes explained as the result of the poisonous effect of a mushroom, but it is not proved. The berserkers were Odin worshippers, and the rage could as well be psychopathic, with the worship of Odin and ecstasy as the main goal.
The berserkers attached themselves to royal and noble courts as bodyguards and shock troops. They formed, among others, the household guard of Norway's King Harald I Fairhair.