This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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adj. Beyond question or doubt. See Synonyms at authentic.

adj. undeniable, obvious

adj. Not questionable.

adj. Not inviting questions or conversation.

That cannot be questioned or doubted; indubitable; certain: as, unquestionable evidence or truth; unquestionable courage.

Averse to being questioned; averse to conversation.

adj. incapable of being questioned

adj. not open to question

adj. not counterfeit or copied

"The Muslim prayer reminded me of what old-school Catholicism must have resembled — steeped in unquestionable tradition and inaccessibility, yet beautiful, a bit magical, and surreal."

for sure    indubitable    mathematical    acknowledged    beyond doubt    unimpeachable    unambiguous    unequivocal    univocal    echt   

ambiguous    conjectural    disputable    questionable   

undeniable   

indubitable    sure    indisputable    questionless    unimpugnable    certain    dead   

questionable   

undoubted    indisputable    undeniable    indubitable    unimpeachable    incontrovertible    unquestioned    unequivocal    unassailable    unswerving