Antonyms of adj indubitable

1 sense of indubitable

Sense 1
beyond doubt (predicate), indubitable -- (too obvious to be doubted)

INDIRECT (VIA unquestionable) -> questionable -- (subject to question; "questionable motives"; "a questionable reputation"; "a fire of questionable origin")

Similarity of adj indubitable

1 sense of indubitable

Sense 1
beyond doubt (predicate), indubitable -- (too obvious to be doubted)
       => unquestionable (vs. questionable) -- (incapable of being questioned; "unquestionable authority")