01 a. 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.
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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.