WordWeb Online

1. Open to doubt or suspicion
   "the candidate's doubtful past";
   - dubious, dubitable, in question, doubtable, questionable
    
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
   "drew a few doubtful conclusions";
   - tentative
    
4. Having a probability too low to inspire belief
   - improbable, unbelievable, unconvincing, unlikely

See also: deniable, implausible, incertain [non-standard], questionable, uncertain, unsettled, unsure

Encyclopedia: Doubtful