Same word as 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