Every time someone goes berserk, they may be invoking a bear. The question of whether they are has kept etymologists arguing for two centuries.
In Old Norse, a berserkr was a warrior who fought in a frenzy. The earliest known written example is in a 9th-century poem by Thorbjorn Hornklofi. Later, Snorri Sturluson described Odin's men as furious like dogs or wolves, biting their shields and as strong as bears.
The second half of the word is clear. Serkr means "shirt" or "coat," related to the obsolete English word sark. The first half is the problem. Most scholars, including those the OED follows, read ber- as "bear," making a berserker a warrior clothed in bearskin. The rival reading takes it as berr, "bare," a man who fought without armor.
That second idea had a famous champion. Walter Scott, in a note to the 1831 edition of The Pirate, said the warriors were "so called from fighting without armour." Etymonline rejects the bare reading outright. Grammarphobia concludes that the true origin may never be known. The bear-shirt reading is the stronger one, but it is not proven.
The word reached English in the early 1800s, and sources disagree on who brought it. Etymonline credits Scott's The Pirate in 1822. Grammarphobia, citing the OED, has Scott using it in 1814. Wordorigins.org found it earlier still, in a January 1800 Edinburgh Magazine summary of a saga, spelled "BERSERKER." It also found Scott writing "Berserkar" in an 1806 ballad note, so he was using the word well before The Pirate.
English then shortened the noun to "berserk," by 1835 according to Etymonline. The adjective followed: 1844 in Etymonline, and 1851 in Charles Kingsley's Yeast by the OED's count. For decades the word kept a capital letter, as a name for a kind of Viking.
The everyday phrase is younger. An 1894 Colorado newspaper has the earliest "going berserk" Grammarphobia found. Rudyard Kipling used "went Berserk" in a comic story in 1895. By 1940 a Chicago Tribune headline could say "America goes berserk," with no Vikings in sight.