Similar word to unconceivable:

doubtful, dubious, far out, farfetched, fishy, flimsy, for the birds, full of holes, impossible, improbable, inconceivable, incredible, obscure, problematic, puzzling, reachy, shaky, suspect, thin, too much, unbelievable, unconvincing, unreasonable, unsubstantial, weak, won't hold water, won't wash