What does Boxing Day have to do with boxing?
mid-12c., wil "wile, trick," perhaps from Old North French *wile (Old French guile), or directly from a Scandinavian source (cf. Old Norse vel "trick, craft, fraud," vela "defraud"). Perhaps ultimately related to Old English wicca "wizard" (see Wicca). Lighter sense of "amorous or playful trick" is from c.1600. Wily is attested from c.1300.