Questionable, a.
1. Admitting of being questioned; inviting, or seeming to invite, inquiry. [R.] Thou com'st in such a questionable shape That I will speak to thee. Shak.
2. Liable to question; subject to be doubted or called in question; problematical; doubtful; suspicious. It is questionable whether Galen ever saw the dissection of a human body.T. Baker.
Syn. -- Disputable; debatable; uncertain; doubtful; problematical; suspicious.
Shades of meaning
Fernald has no entry of its own for questionable; he lists it under equivocal.
Where he writes about it:
Questionable may be used nearly in the sense either of dubious or of doubtful; a questionable statement is one that must be proved before it can be accepted. — from equivocal
To say that one's honesty is questionable is a mild way of saying that in the opinion of the speaker he is likely to prove dishonest. — from equivocal