UNTHINKABLE

adj. 1 inconceivable, unbelievable, unimaginable, incredible, incomprehensible, beyond belief, extraordinary, Colloq mind-boggling, Slang mind-blowing It seemed unthinkable that such a mild-mannered man could have been a mass murderer 2 unacceptable, absurd, illogical, impossible, improbable, unlikely, out of the question, preposterous, ridiculous, laughable, ludicrous, Colloq not on It is simply unthinkable for her to go to the ball unattended