doubt·ful / ˈdoutfəl/ • adj. 1. feeling uncertain about something: he looked doubtful, but gave a nod | I was doubtful of my judgment. 2. not known with certainty: the fire was of doubtful origin. ∎ improbable: it is doubtful whether these programs have any lasting effect. ∎ not established as genuine or acceptable: of doubtful legality. DERIVATIVES: doubt·ful·ly adv. doubt·ful·ness n.