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.