If you are doubtful about something, you feel unsure or uncertain about it. I was still very doubtful about the chances for success Why did he sound so doubtful? = dubious + doubtfully doubt·ful·ly Keeton shook his head doubtfully. = dubiously
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"
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
disapproval If you say that something is of doubtful quality or value, you mean that it is of low quality or value. selling something that is overpriced or of doubtful quality = dubious
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
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
If it is doubtful that something will happen, it seems unlikely to happen or you are uncertain whether it will happen. For a time it seemed doubtful that he would move at all It is doubtful whether Tweed, even with his fluent French, passed for one of the locals