(a.) Not credible; not likely to be believed\; not wearing the appearance of truth or credibility.

a bit thick a bit thin absurd beyond belief debatable doubtable doubtful dubious dubitable fishy flimsy hard of belief hard to believe hardly possible improbable inconceivable incredible more than doubtful not deserving belief open to doubt open to suspicion passing belief preposterous problematic puzzling questionable ridiculous staggering belief suspect suspicious tall thick thin unbelievable unconvincing unearthly ungodly unimaginable unlikely unpromising unreasonable unsubstantial unthinkable unworthy of belief weak