In Old Norse writings, berserkers are warriors who fight in a furious, uncontrollable, possibly drug-induced trance that gives them great strength and courage. The word came to English in the early 19th century and was initially used mainly in reference to the warriors, but it was soon shortened to berserk and gained its secondary, now more common meaning: frenetically upset or violent. The adjective is often embedded in the verb phrase go berserk, similar in construction and meaning to phrases such as go crazy and go insane.
Berserk is more closely synonymous with wild and frantic than with crazy and insane, which are broad enough to cover behaviors that are only moderately out of the ordinary. A berserk state is an extreme one, and the berserk person is not easily calmed.
With these examples, we try to roughly trace the development of berserk from its earlier, warrior-related sense to its more modern sense having to do with violent frenzy:
Before they parted Heidreker drew out his sword to look at and admire it; but scarcely did the rays of light fall on the magic blade, when the Berserker rage came on its owner, and he slew his gentle brother. [The Fairy Mythology, Thomas Keightley (1833)]
“But fancy Roland going Berserk! I would never have dreamt of that’.” [Stretton: A novel, Henry Kingsley (1869)]
There were some eminently sane Scotsmen who could no more restrain a kind of berserk fury at the mention of the last rebellion than they could restrain their tears at a reading of Burns’s “Auld Lang Syne.” [A Prince of Romance, Stephen Chalmers (1911)]
A theory that James Clinstock, 38-year-old massive Indian wrestler, was strangled with a towel In a dental laboratory after he went berserk and “acted like Frankenstein” was advanced by the state today. [AP via Lewiston Morning Tribune (1944)]
Worried shrinks want to find out why a 3-year-old boy went berserk and savagely attacked a playmate — stabbing him several times with a butcher knife. [Weekly World News (1990)]
Television images showed berserk buyers charging through doors on what is known as “Black Friday,” the day after the Thanksgiving holiday that is considered crucial for retailers’ annual earnings. [News.com.au (2012)]