doubtful (adj.)

late 14c., "causing doubt, not distinct in character, meaning, or appearance," from doubt (n.) + -ful. From c. 1400 as "of uncertain issue, precarious." From early 15c. as "full of doubt, having doubt, hesitant, wavering." By mid-15c. as "admitting or subject to doubt." Related: Doubtfully; doubtfulness.

Other words that have been used in English in some or all of these senses were doubtous "undetermined" (mid-14c.); doutive "filled with doubt" (late 14c.); douty "ambiguous, enigmatic, obscure" (late 14c.); doubtable (c. 1400); doubtsome (1510s).

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Definitions of doubtful from WordNet

doubtful (adj.)
open to doubt or suspicion; "it was more than dubitable whether the friend was as influential as she thought"- Karen Horney;
the candidate's doubtful past
Synonyms: dubious / dubitable / in question
doubtful (adj.)
fraught with uncertainty or doubt;
they were doubtful that the cord would hold
it was doubtful whether she would be admitted
Synonyms: dubious
doubtful (adj.)
unsettled in mind or opinion;
Synonyms: tentative