Im‐prob″a‐ble (?), a. [L. improbabilis; pref. im- not + probabilis probable: cf. F. improbable. See Probable.] Not probable; unlikely to be true; not to be expected under the circumstances or in the usual course of events; as, an improbable story or event.
He... sent to Elutherius, then bishop of Rome, an improbable letter, as some of the contents discover. Milton.
— Im‐prob″a‐ble‐ness, n. — Im‐prob″a‐bly, adv.