The word "unquestionable" is used to describe something that cannot be doubted or disputed. Its opposite, or antonym, is "questionable," which describes something that is open to doubt, suspicion, or controversy. Another antonym for "unquestionable" is "doubtful," which means uncertain or unsure. Other possible antonyms include "ambiguous," "equivocal," "debatable," "unclear," and "vague," each of which suggests different levels of uncertainty or ambiguity. When choosing an antonym for "unquestionable," it's important to consider the context and intended meaning, as well as any subtle connotations or implications of the chosen antonym.