doubt ‧ ful /ˈdaʊtf ə l/ BrE AmE adjective
[ Word Family: adjective : ↑ doubtful , doubtless, ↑ indubitable , ↑ undoubted ; adverb : ↑ doubtfully , ↑ indubitably , ↑ undoubtedly ; noun : ↑ doubt , ↑ doubter ; verb : ↑ doubt ]
1 . 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.
2 . not sure that something is true or right:
‘Everything’s going to be all right, you’ll see.’ Jenny looked doubtful.
doubtful if/whether
I’m still doubtful whether I should accept this job.
doubtful about (doing) something
At first we were doubtful about employing Charlie.
3 . unlikely to be successful:
Already the whole scheme was looking increasingly doubtful.
4 . probably not good SYN dubious :
Here the tap water is of doubtful quality.
—doubtfully adverb