The term berserk comes from the old Norse meaning 'bear shirt' or 'bear skin'. Frequently men prior to going into battle would eat mushrooms that contained psychoactive alkaloids. These mushrooms first would have a narcotic effect where the warrior would fall asleep, upon waking there was a sense of vigor, superhuman strength and fearlessness. They would go into battle in this state, hence the name berserk. Under the influence of these mushrooms the men were formidable fighters and this lent to the legend of Norsemen as generally all around tough guys for a relatively localized culture.