Doubtful, a.
1. 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. Methinks I should know you, and know this man; Yet I am doubtful. Shak. With doubtful feet and wavering resolution. Milton.
2. 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. Beauty is but a vain and doubtful good. Shak. Is it a great cruelty to expel from our abode the enemy of our peace, or even the doubtful friend [i. e., one as to whose sincerity there may be doubts] Bancroft.
3. Characterized by ambiguity; dubious; as, a doubtful expression; a doubtful phrase.
4. Of uncertain issue or event. We . . . have sustained one day in doubtful fight. Milton. The strife between the two principles had been long, fierce, and doubtful. Macaulay.
2 more senses
5. Fearful; apprehensive; suspicious. [Obs.] I am doubtful that you have been conjunct And bosomed with her. Shak.
Syn. -- Wavering; vacillating; hesitating; undetermined; distrustful; dubious; uncertain; equivocal; ambiguous; problematical; questionable.
Shades of meaning
Fernald has no entry of its own for doubtful; he lists it under equivocal, obscure, 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