What does Boxing Day have to do with boxing?
1570s, probably from a dialectal survival of Old English wiðer "against, contrary, opposite" (see with) + plural suffix. Possibly so called because the withers are the parts of the animal that oppose the load. Cf. German Widerrist "withers," from wider "against" + Rist "wrist."
1530s, alteration of Middle English wydderen "dry up, shrivel" (c.1300), apparently a differentiated and special use of wederen "to expose to weather" (see weather). Cf. German verwittern "to become weather-beaten," from Witter "weather."