(a.) Not credible; surpassing belief; too extraordinary and improbable to admit of belief; unlikely; marvelous; fabulous.

a bit thick a bit thin absurd amazing antic appalling astonishing astounding awe-inspiring awesome beguiling bewildering beyond belief bizarre cockamamie conspicuous crazy doubtable doubtful dubious dubitable egregious enigmatic exceptional extraordinary extravagant fabulous fanciful fantastic fascinating fictitious flimsy foolish formidable grotesque hard of belief hard to believe hardly possible high-flown humongous implausible impossible improbable incomprehensible inconceivable laughable ludicrous marked marvelous miraculous monstrous more than doubtful mythical nonsensical not deserving belief notable noteworthy noticeable of mark open to doubt open to suspicion outlandish outrageous outre outstanding passing belief passing strange phenomenal poppycockish preposterous problematic prodigious puzzling questionable rare remarkable ridiculous sensational signal staggering belief strange striking stupendous superior suspect suspicious tall thick thin tremendous unaccountable unbelievable uncommon unconvincing unearthly unexpected ungodly unheard-of unimaginable unique unlikely unprecedented unpromising unreal unrealistic unsubstantial untenable unthinkable unworthy of belief weak weird wild wonderful wondrous