I’m going to let the OED speak on this one:
Etymology: < Icelandic berserkr, accusative berserk, plural -ir, of disputed etymology; Vigfusson and Fritzner show that it was probably = ‘bear-sark,’ ‘bear-coat.’A wild Norse warrior of great strength and ferocious courage, who fought on the battle-field with a frenzied fury known as the ‘berserker rage’; often a lawless bravo or freebooter. Also fig. and attrib. Now usu. as adj., frenzied, furiously or madly violent; esp. in phr. to go berserk .
There aren’t many English words that come from Icelandic, but those we do have are awesome.