Synonyms for Doubtful (same or very similar meaning)
WordNet sense 1 (unsettled in mind or opinion):
tentative
WordNet sense 2 (still in doubt):
unsettled
WordNet sense 3 (open to doubt or suspicion):
dubious, dubitable
WordNet sense 4 (subject to question):
questionable
WordNet sense 5 (lacking or indicating lack of confidence or assurance):
uncertain, unsure
From the ODE community, based on WordNet
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