Now used as an adjective to mean crazy and off the wall, its etymology is as a noun. Berserk has its roots in old Norse languages and refers to a Scandinavian in battle who was invulnerable. A band of them was called berserkers or berserkergang. Some historians have posited that the groups of warriors would use naturally occurring, hallucinogenic drugs before heading into battle, thus their unusually frenzied state.
They wore the hides of bears (in Norse, bera=bear and serkr=shirt), gnawed on their metal shields, foamed at the mouth. By 1818 (according to Merriam-Webster), berserk had come to simply refer to a person who acts with reckless abandon.
There are berserk potions and “power-ups” in many video games with Medieval and magic themes. It is also a long running manga series. The main character is a mercenary warrior in Medieval Europe.
Maybe one of the last modern usages of the term was as the title of a Joan Crawford picture in 1967. Desperately clinging to her reputation and falling beauty, the film follows the desperate (and unfeeling) owner/ringmistress of Rivers’ Circus as one by one, circus performers turn up dead. Berserk! uses its title to imply the unruly, strange and unpredictable underworld 
of a traveling circus.