Source : Webster's Revised Unabridged Dictionary (1913)

Doubtful \Doubt"ful\, 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.

Source : WordNet®

doubtful
     adj 1: open to doubt or suspicion; "the candidate's doubtful past";
            "he has a dubious record indeed"; "what one found
            uncertain the other found dubious or downright false";
            "it was more than dubitable whether the friend was as
            influential as she thought"- Karen Horney [syn: {dubious},
             {dubitable}, {in question(p)}]
     2: fraught with uncertainty or doubt; "they were doubtful that
        the cord would hold"; "it was doubtful whether she would
        be admitted"; "dubious about agreeing to go" [syn: {dubious}]
     3: unsettled in mind or opinion; "drew a few tentative
        conclusions" [syn: {tentative}]