What does Boxing Day have to do with boxing?
mid-13c., "theft, action or practice of stealing," from Old English *stælþ, which is related to stelen (see steal (v.)), from Proto-Germanic *stælitho (cf. Old Norse stulþr), with Proto-Germanic abstract noun suffix *-itho (see -th (2)). Sense of "secret action" developed c.1300, but the word also retained its etymological sense into 18c. Got a boost as an adjective from stealth fighter, stealth bomber, radar-evading U.S. military aircraft, activated 1983.
A great bargain: I got that for half price, a real steal (1940s+)