Berserks, or berserkers, were wild warriors who fought savagely in battle and worshiped Odin, king of the Norse* gods. The word berserk comes either from a Norse word for "bear-shirt" (meaning bearskin) or from "bare-of-shirt" (meaning without mail or armor). Either definition would be appropriate because berserks entered battle without armor and dressed in animal skins.
Some people believed that berserks could become man-wolves or man-bears. The legend of the werewolf may have grown from this tradition. It was said that berserks howled and fought with the fury of wild animals and bit their shields during battle. Their extraordinary strength enabled them to overcome their enemies. Neither iron nor fire could hurt them. Because of their fierceness and skill in battle, berserks sometimes served as bodyguards or special troops for royal households. However when not fighting, berserks often went wild, raping and robbing members of Norse communities. From such behavior, we get the phrase "going berserk."