In Norse mythology, there was a famous, furious fighter who scorned the use of heavy mail, entering battle without armor, thus acquiring his name, Berserker, or “Bear Shirt.”
It was said of him that he could assume the form of wild beasts, and that neither iron nor fire could harm him, for he fought with the fury of wild beasts and his foes were unable to touch him.
Each of his twelve sons also carried the name Berserker, and each was as furious a fighter as the father. From these legendary heroes the early Norse described any fierce fighter as a “berserker,” especially one so inflamed with the fury of fighting that he was equally dangerous to friend and foe.
So, since the nineteenth century, we have adopted the term and say of anyone in a furious rage that he has “gone berserk,” using it synonymously with “run amuck.”