inconceivable

too extraordinary or improbable to believe the formerly inconceivable idea that humans could land on the moon Synonyms fantastic (also fantastical), implausible, incredible, incredulous, unbelievable, uncompelling, unconceivable, unconvincing, unimaginable, unthinkableRelated Words debatable, disputable, dodgy [chiefly British], doubtable, doubtful, dubious, dubitable, far-fetched, fishy, flimsy, questionable, shaky, suspect, suspicious, unlikely, unreasonable; hopeless, impossible; absurd, comical, farcical, laughable, ludicrous, outlandish, preposterous, ridiculous, risible, silly; indefensible, insupportable, untenableNear Antonyms likely, possible, probable; reasonable; certain, incontestable, indisputable, indubitable, questionless, sure, undeniable, undoubted, unquestionableAntonyms believable, cogitable, conceivable, convincing, credible, creditable, imaginable, plausible, supposable, thinkable