"strong, brave, spirited, valiant," Middle English doughti, from 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- (source also of Middle High German thtec, German tüchtig "efficient, capable," Middle Dutch duchtich "large, sturdy, powerful," Danish dygtig "virtuous, proficient," Gothic daug "is fit"), from PIE *dheugh- "to be fit, be of use, proper; meet, hit the mark" (source also of Sanskrit duh "gives milk;" Greek teukhein "to manufacture, accomplish; make ready;" Irish dual "becoming, fit;" Russian duij "strong, robust;" German Tugend "virtue").
Rare after 17c.; in deliberately archaic or mock-heroic use since c. 1800. If it had survived in living language, its modern form would be dighty.