What does Boxing Day have to do with boxing?
Old English dohtig "competent, good, valiant," from dyhtig "strong," related to dugan "to be fit, be able, be strong," and influenced by its past participle, dohte.
All from Proto-Germanic *duhtiz- (cf. Middle High German tühtec, German tüchtig, Middle Dutch duchtich), from PIE *dheugh- "to be fit, be of use, proper" (cf. German Tugend "virtue," Greek teukhein "to make ready," Irish dual "becoming, fit," Russian dužij "strong, robust"). Rare after 17c.; in deliberately archaic or mock-heroic use since c.1800. If it had survived, its modern form would be dighty.