Meaning of “doubtful” - Learner’s Dictionary

doubtful

adjective uk us /ˈdaʊtfəl/
NOT PROBABLE

B2 not probable:

It's doubtful if/whether he'll be able to come.
[ + (that) ] It's doubtful that anyone survived the fire.
NOT CERTAIN

B2 not feeling certain about something:

She's still doubtful about her decision.
doubtfully adverb

(Definition of “doubtful” from the Cambridge Learner’s Dictionary © Cambridge University Press)