Noah Webster's New International Dictionary of the English Language

1. (a.) 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.

2. (a.) 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.

3. (a.) Characterized by ambiguity; dubious; as, a doubtful expression; a doubtful phrase.

4. (a.) of uncertain issue or event.

5. (a.) Fearful; apprehensive; suspicious.