An article by Peter Barglow says this.
"The violence called “Going Berserk” has an ancient history. “Berserkers” were Norse warriors, “Baresaks,” who battled, often naked, in an uncontrollable, trance-like bestial fury, first mentioned in the late ninth century Scandinavian poem, Haraldskvaedi. (“Baresark” may refer to a medieval thin shirt made from a bear pelt.)"
