What does Boxing Day have to do with boxing?
c.1200, "to strike, hit, beat, knock;" c.1300, "to deprive of life," perhaps from an unrecorded variant of Old English cwellan "to kill" (see quell), but the earliest sense suggests otherwise. Sense in to kill time is from 1728. Related: Killed; killing. Kill-devil, colloquial for "rum," especially if new or of bad quality, is from 1630s.
early 13c., "a stroke, a blow," from kill (v.). Meaning "act of killing" is from 1814; that of "a killed animal" is from 1878. Lawn tennis serve sense is from 1903. The kill "the knockout" is boxing jargon, 1950.
"stream," 1630s, American English, from Dutch kil, from Middle Dutch kille "riverbed," especially in place names (e.g. Schuylkill). A common Germanic word, the Old Norse form, kill, meant "bay, gulf" and gave its name to Kiel Fjord on the German Baltic coast and thence to Kiel, the port city founded there in 1240.