Adjective doubtful has 3 senses
  1. doubtful, dubious, dubitable, in question - 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
    Antonym: unquestionable (indirect, via questionable)
  2. doubtful, dubious - 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"
    Antonyms: certain, sure (indirect, via uncertain, unsure)
    Antonyms: certain, sure (indirect, via uncertain, unsure)
  3. doubtful, tentative - unsettled in mind or opinion; "drew a few tentative conclusions"
    Antonym: settled (indirect, via unsettled)