From BiblePortal Wikipedia

Webster's Dictionary [1]

(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.) Fearful; apprehensive; suspicious.

(5): ( a.) Of uncertain issue or event.

King James Dictionary [2]

Doubtful, a.

1. Dubious not settled in opinion undetermined wavering hesitating applied to persons as, we are doubtful of a fact, or of the propriety of a measure. 2. Dubious ambiguous not clear in its meaning as a doubtful expression. 3. Admitting of doubt not obvious, clear or certain questionable not decided as a doubtful case a doubtful proposition it is doubtful what will be the event of the war. 4. Of uncertain issue.

We have sustained one day in doubtful fight.

5. Not secure suspicious as, we cast a doubtful eye. 6. Not confident not without fear indicating doubt.

With doubtful feet, and wavering resolution.

7. Not certain or defined as a doubtful hue.