What does Boxing Day have to do with boxing?
early 14c., of uncertain origin, perhaps from Scandinavian (cf. Swedish dialectal puta "to be puffed out"), or Frisian (cf. East Frisian püt "bag, swelling," Low German puddig "swollen"); related via notion of "inflation" to Old English ælepute "fish with inflated parts," and Middle Dutch puyt, Flemish puut "frog," from hypothetical PIE imitative root *beu- suggesting "swelling" (see bull (n.2)). Related: Pouted; pouting. As a noun from 1590s.