From Wikipedia: Berserkers (or berserks) were champion Norse warriors who are primarily reported in the Old Norse literature to have fought in a nearly uncontrollable, trance-like fury, a characteristic which later gave rise to the English word berserk. These Viking champions would often go into battle without mail-coats. To "go berserk" was to "hamask", which translates as "change form", in this case, as with the sense "enter a state of wild fury".