sark n.

Middle English shirt, shirte, "garment for the upper body worn next to the skin," from Old English scyrte, from Proto-Germanic *skurtjon "a short garment" (source also of Old Norse skyrta, Swedish skjorta "skirt, kirtle;" Middle Dutch scorte, Dutch schort "apron;" Middle Low German schörte, Middle High German schurz, German Schurz "apron"), which is perhaps related to Old English scort, sceort "short," etc., from PIE root *sker- (1) "to cut," on the notion of "a cut piece."

OED notes that "the meaning of the word in OE. is obscure, as the only instance of its occurrence is a gloss in which the meaning of the Latin word was probably not understood." Lithuanian šarkas "shirt," Old Church Slavonic sraka "tunic," Russian soročka, Finnish sarkki "shirt" perhaps are from Germanic.

Formerly of the chief under-garment worn by both men and women, but in modern use it has long been only that for men; in reference to women's tops, the word was reintroduced 1896.

Bloody shirt, a blood-stained shirt exposed as a symbol of some outrage, to arouse indignation or resentment, is attested from 1580s, usually figurative. Shirt since late 14c. often has been figurative of one's goods or possessions, hence give (someone) the shirt off one's back (1771); lose one's shirt "suffer total financial loss" (1935). To keep one's shirt on "be patient" (1904) is from the notion of (not) stripping down for a fight.

1844, from berserk (n.) "Norse warrior" (by 1835), an alternative form of berserker, a word which was introduced (as berserkar) by Sir Walter Scott in "The Pirate" (1822), from Old Norse berserkr (n.) "raging warrior of superhuman strength." It is probably from *ber- "bear" + serkr "shirt," thus literally "a warrior clothed in bearskin" (see bear (n.) + sark). Thus not, as Scott evidently believed, from Old Norse berr "bare, naked" and meaning "warrior who fights without armor."

Thorkelin, in the essay on the Berserkir, appended to his edition of the Kristni 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]

Perhaps later writers mistook the -r for an agent-noun suffix. The picture is further complicated because it has the form of the Old Norse plural, and English berserker sometimes is plural. The adjectival use probably grew from such phrases as berserk frenzy, or as a title (Arngrim the Berserk).

1640s, from under (adj.) + shirt (n.). Similar formation in North Frisian onnersjürt, Danish underskjorte. Old English had undersyrc (see sark (n.)).