What does Boxing Day have to do with boxing?
Old English scyrte "skirt, tunic," from Proto-Germanic *skurtjon "a short garment" (cf. Old Norse skyrta, Swedish skjorta "skirt, kirtle;" Middle Dutch scorte, Dutch schort "apron;" Middle High German schurz, German Schurz "apron"), related to Old English scort, sceort "short," from PIE *(s)ker- (1) "to cut" (see shear (v.)).
Formerly of the chief garment worn by both sexes, but in modern use long only of that for men; in reference to women's tops, reintroduced 1896. Bloody shirt, exposed as a symbol of outrage, is attested from 1580s. To give (someone) the shirt off one's back is from 1771. To lose one's shirt "suffer total financial loss" is from 1935. To keep one's shirt on "be patient" (1904) is from the notion of (not) stripping down for a fight.
any of a variety of cloth garments having sleeves and worn on the upper part of the body, often under a coat, jacket, or other garment. Shirts were worn as early as the 18th dynasty of ancient Egypt; they were made of a rectangular piece of linen, folded and sewn up the sides, with openings left for the arms and a hole cut at the fold for the head. There are also shirts preserved from ancient Egypt that have long, tight sleeves sewn into the armholes.