What does Boxing Day have to do with boxing?
Old English stif "rigid, inflexible," from Proto-Germanic *stifaz "inflexible" (cf. Dutch stijf, Old High German stif, German steif "stiff;" Old Norse stifla "choke"), from PIE *stipos-, from root *steip- "press together, pack, cram" (cf. Sanskrit styayate "coagulates," stima "slow;" Greek stia, stion "small stone," steibo "press together;" Latin stipare "pack down, press," stipes "post, tree trunk;" Lithuanian stipti "stiffen," stiprus "strong;" Old Church Slavonic stena "wall"). Of battles and competitions, from mid-13c.; of liquor, from 1813. To keep a stiff upper lip is attested from 1815.
"corpse," 1859, slang, from stiff (adj.) which had been associated with notion of rigor mortis since c.1200. Meaning "working man" first recorded 1930, from earlier genitive sense of "contemptible person" (1882). Slang meaning "something or someone bound to lose" is 1890 (originally of racehorses), from notion of "corpse."
"fail to tip," 1939, originally among restaurant and hotel workers, probably from stiff (n.) in slang sense of "corpse" (corpses don't tip well, either). Extended by 1950 to "cheat."
[the underworld senses having to do with forged and clandestine papers, cheating, etc, are derived fr an early 1800s British sense, ''paper, a document,'' probably based on the stiffness of official documents and document paper; the senses having to do with failure, etc, are related to the stiffness of a corpse; the sense of harsh snubbing, etc, is fr the stiff-arm in football, where a player, usually a runner, straightens out his arm and pushes it directly into the face or body of an intending tackler]