WordWeb Online

1. Open to doubt or suspicion
   "the candidate's doubtful past"
   - dubious, dubitable, in question, doubtable
    
2. Fraught with uncertainty or doubt
   "they were doubtful that the cord would hold"; "it was doubtful whether she would be admitted"
   - dubious
    
3. Unsettled in mind or opinion
   - tentative

See also: doubtfulness, incertain, questionable, uncertain, unsettled, unsure