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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.