Several sources including Phrase Finder explain a ‘Berserker’ was a Viking warrior who fought with wild ferocity. “The berserker fighting tradition, in which the warriors took on the spirit (or even in their belief, the shape) of bears whilst foaming at the mouth and gnawing at the edge of their shield, is the source of the Vikings’ fierce tradition.” It is believed the word is derived from “bear sark,” which means “bear coat.” The Vikings would show their bravery by going into battle with their bear sark jackets open. Sir Walter Scott wrote in 1822 that “The berserkers were so called from fighting without armour.”