That is doubtful which is fairly open to doubt; that is dubious which has become the subject of doubts so grave as scarcely to fall short of condemnation; as, a dubious reputation.

J. C. Fernald, English Synonyms and Antonyms (1914)

Example sentences

uncertain Tom told me that he's uncertain what he should do.
questionable There are many questionable points in this ledger.
dubious He also brought out one more dubious conclusion.
equivocal Research in this area is somewhat equivocal.
tentative The parties have reached a tentative agreement.
improbable It seemed improbable that Tom would succeed.

Popularity over time

Word	1850 → 2019	Trend
doubtful		falling
uncertain		steady
questionable		falling
dubious		steady
equivocal		steady
tentative		falling

Use in printed books, per decade from 1850 to 2019.

By length

5 letters: vague¹, dicey²
6 letters: unsure²
7 letters: suspect², obscure², dubious³
9 letters: uncertain³, skeptical³, ambiguous⁴, tentative³, unsettled³, undecided⁴, debatable⁴, sceptical, equivocal⁴
10 letters: suspicious³, precarious⁴, improbable⁴, hesitating⁴, apocryphal⁴, disputable⁴
11 letters: problematic⁴, distrustful³
12 letters: questionable⁴
13 letters: indeterminate⁵, problematical⁵

The small number after each word is its number of syllables (dicey² = 2 syllables).

In other languages

Français: douteux
Italiano: dubitabile
Español: dudoso
Deutsch: zweifelhaft
Català: dubtós