Not settled in opinion; undetermined; wavering; hesitating in belief; also used, metaphorically, of the body when its action is affected by such a state of mind; as, we are doubtful of a fact, or of the propriety of a measure.
Admitting of doubt; not obvious, clear, or certain; questionable; not decided; not easy to be defined, classed, or named; as, a doubtful case, hue, claim, title, species, and the like.
Characterized by ambiguity; dubious; as, a doubtful expression; a doubtful phrase.
Although a skillful flatterer is a most delightful companion if you have him all to yourself, his taste becomes very doubtful when he takes to complimenting other people. Charles Dickens
A civilized society is one which tolerates eccentricity to the point of doubtful sanity. Robert Frost
Life is like music, it must be composed by ear, feeling and instinct, not by rule. Nevertheless one had better know the rules, for they sometimes guide in doubtful cases, though not often. Paracelsus
I remember breaking the news to both my parents that I wanted to be a director, and they both looked very doubtful. They didn't know what a closet Hindi film buff I was. I used to dance to old Hindi films songs on the sly, so my decision to be a part of Hindi cinema was shocking even for my parents. Karan Johar
The tendency of modern scientific teaching is to neglect the great books, to lay far too much stress upon relatively unimportant modern work, and to present masses of detail of doubtful truth and questionable weight in such a way as to obscure principles. Ronald Fisher