Unbelievable

unimaginable impossible incredible implausible doubtful questionable far-fetched astonishing staggering outlandish preposterous unthinkable inconceivable dubious flimsy weak fishy suspect phony unconvincing thin reaching past belief beyond belief full of holes open to doubt improbable for the birds lamebrained harebrained scatterbrained cockamamie cockeyed screwy kooky thick incogitable won’t wash won’t hold water