Unbelievable

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