Dubious, a.
Etymology: [L. dubius, dubiosus, fr. duo two. See Two, and cf. Doubt.]
1. Doubtful or not settled in opinion; being in doubt; wavering or fluctuating; undetermined. "Dubious policy." Sir T. Scott. A dubious, agitated state of mind. Thackeray.
2. Occasioning doubt; not clear, or obvious; equivocal; questionable; doubtful; as, a dubious answer. Wiping the dingy shirt with a still more dubious pocket handkerchief. Thackeray.
3. Of uncertain event or issue; as, in dubious battle.
Syn. -- Doubtful; doubting; unsettled; undetermined; equivocal; uncertain. Cf. Doubtful.
Shades of meaning
Fernald has no entry of its own for dubious; he lists it under equivocal, precarious.
Where he writes about it:
That is doubtful which is fairly open to doubt; that is dubious which has become the subject of doubts so grave as scarcely to fall short of condemnation; as, a dubious reputation. — from equivocal
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