un conceivable (adjective) unconceivable: implausible
Sinonime: 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