Webster's Revised Unabridged Dictionary · 1913

Doubtful

Doubtful , adjective

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. — Shakespeare
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. — Shakespeare
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
5.
Fearful; apprehensive; suspicious. [Obsolete]
I am doubtful that you have been conjunct And bosomed with her. — Shakespeare