To "go berserk" is to lose self-control in a violent or frenzied way. The word comes directly from Old Norse berserkr, the name of an elite (and feared) class of Viking warrior. The traditional reading of the word is "bear shirt," from björn ("bear") and serkr ("shirt"). A berserker fought wearing a bearskin — or, in some accounts, fought in such a state of fury that he was thought to be a bear in human form.
Frenzy in the Sagas
The Old Norse sagas describe berserkers entering a battle-trance called the berserkergang — a state of howling, biting fury in which they were said to feel no pain and fight with superhuman strength. Modern scholars have suggested various explanations: ritual psychology, ergotism (a fungal poisoning), the consumption of mushrooms or alcohol, or simple literary embellishment. Whatever the truth, the berserker became a figure of dread in Viking-Age stories.
Bear or Bare?
Some scholars have proposed an alternative reading of the first element: berr in Old Norse can also mean "bare," and a berserker might originally have been a "bare-shirt" warrior — one who fought without armour, naked from the waist up. The mainstream view still favours "bear-shirt," but the ambiguity is real, and a few medieval Icelandic sources play on both possibilities.
Into English (Late, via Iceland)
"Berserk" is one of the more recent Norse borrowings in English. It was popularised in the early 19th century by Sir Walter Scott, who picked it up from Icelandic saga sources. From Scott's historical novels it spread into general English use, first as a noun (a "berserker") and then as an adjective ("to go berserk"). The modern adjective is now used loosely for any wild, uncontrolled behaviour — the bearskin and the battle-frenzy quietly forgotten.