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doubt‧ful

1 probably not true or not likely to happen :
Prospects for a lasting peace remain doubtful.
Prospects for a lasting peace remain doubtful.2 not sure that something is true or right :
3 unlikely to be successful :
Already the whole scheme was looking increasingly doubtful.
Already the whole scheme was looking increasingly doubtful.4 probably not good [= dubious]:
Here the tap water is of doubtful quality.
Here the tap water is of doubtful quality. —doubtfully adverb
