What does "improbable" mean?

Improbable: 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.