Shirt and skirt are the same word. Old English scyrte gave shirt by the regular softening of sc- to sh-; Old Norse skyrta gave skirt, with the hard sk- that Norse kept. English borrowed the Norse form during the Danish settlement, ended up with two versions of one word, and did the sensible thing — assigned one to the top half of the body and one to the bottom.
That pair is the cleanest evidence available for how Norse borrowings work, and the test generalises: ship and skipper, shatter and scatter, shriek and screech. Underneath both is a Germanic word for a short garment, probably related to Old English scort, "short," from a root meaning "to cut" — a shirt being, originally, a cut piece. The Old English evidence is thin: the word appears once, in a gloss whose writer may not have understood the Latin he was translating.
shirt (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.
shirt came in through Old English, and its spelling follows that family's conventions.
How that family writes these sounds
| /ʃ/ | shship, sheep, shirt |
Old English in the spelling guide · guess another word's family