Learner's definition of DOUBTFUL
[more doubtful; most doubtful]
uncertain or unsure about something
I tried to reassure them, but they remained doubtful.
She gave him a doubtful look. [=a look that shows doubt]
— often + about or of
not likely to be true
— often + that, if, or whether
It's highly doubtful [=unlikely, improbable] that anyone will notice.
It is doubtful if she really meant what she said.
The situation is bad and it's doubtful whether it will get better soon.
likely to be bad
not worthy of trust
unknown or undecided