Our word berserk comes from the beserkers of the Ancient Norse, who were reputed to go into a wild frenzy during battle and, without battle, cut down everything in their path. In English, the word berserker was introduced by Sir Walter Scott in his 1822 novel The Pirate. Scott borrowed this directly from Old Norse berserkr, which is probably a portmanteau of bjorn, meaning "bear", and serkr, meaning "coat". This "bear coat" meaning is a bit contested; some think that it was the equivalent of bare shirt, but because of similar words with connotations of wolves and boars, this has been largely discredited. It really is just to imply the raw savagery of these warriors. Bjorn, through Proto-Germanic bernuz, comes from Proto-Indo-European bruhn, which meant "grey" or "brown". Serkr, through Proto-Germanic sarkiz, comes from PIE ser, which meant "to string" or "attach". So, even further back, a berserker is a "brown attachment".
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.