Quiz: Remember the definition of mal de mer?
alternative form of berserk (q.v.), from Old Norse berserkr, accusative of berserk. This is the oldest form of the word in its revival in Modern English (1822), and perhaps Scott, who introduced it, mistook the -r for an agent-noun suffix. Further compicated because it has the form of the Old Norse plural, and English berserker sometimes is plural.
1844, from berserk (n.) "Norse warrior," by 1835, an alternative form of berserker (1822), a word which was introduced by Sir Walter Scott, from Old Norse berserkr (n.) "raging warrior of superhuman strength;" probably from *ber- "bear" + serkr "shirt," thus literally "a warrior clothed in bearskin." Thus not from Old Norse berr "bare, naked."
Thorkelin, in the essay on the Berserkir, appended to his edition of the Krisini Saga, tells that an old name of the Berserk frenzy was hamremmi, i.e., strength acquired from another strange body, because it was anciently believed that the persons who were liable to this frenzy were mysteriously endowed, during its accesses, with a strange body of unearthly strength. If, however, the Berserk was called on by his own name, he lost his mysterious form, and his ordinary strength alone remained. ["Notes and Queries," Dec. 28, 1850]The adjectival use probably is from such phrases as berserk frenzy, or as a title (Arngrim the Berserk).