What does Boxing Day have to do with boxing?
late 14c., incorporacioun, "act or process of combining of substances; absorption of light or moisture," from Old French incorporacion or directly from Late Latin incorporationem (nominative incorporatio), noun of action from past participle stem of incorporare (see incorporate). Meaning "the formation of a corporate body" (such as a guild) is from early 15c.
Incorporation, n. The act of uniting several persons into one fiction called a corporation, in order that they may be no longer responsible for their actions. A, B and C are a corporation. A robs, B steals and C (it is necessary that there be one gentleman in the concern) cheats. It is a pundering, thieving, swindling corporation. But A, B and C, who have jointly determined and severally executed every crime of the corporation, are blameless. [Ambrose Bierce, 1885]