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.
"to withdraw (a player) from the varsity team to add a year to his or her eligibility," 1950, in reference to the red shirts worn by athletes on the scrimmage squad; from red (adj.1) + shirt (n.). Also as a noun, "a college athlete whose course of study is extended for the sake of sports eligibility" (by 1970). Earlier a red-shirt was "a supporter of Garibaldi" (1860s); hence, generally, "a revolutionary."