probably not true or not likely to happen:
Prospects for a lasting peace remain doubtful.
it is doubtful if/whether
It was doubtful whether the patient would survive the operation.
it is doubtful that
It is doubtful that the missing airmen will ever be found.
not sure that something is true or right:
'Everything's going to be all right, you'll see.' Jenny looked doubtful.
I'm still doubtful whether I should accept this job.
doubtful about (doing) something
At first we were doubtful about employing Charlie.
unlikely to be successful:
Already the whole scheme was looking increasingly doubtful.
probably not good [= dubious]:
Here the tap water is of doubtful quality.