improbable Meaning, Definition & Usage

  1. adjective not likely to be true or to occur or to have occurred
    unlikely.
    • legislation on the question is highly unlikely
    • an improbable event
  2. adjective satellite having a probability too low to inspire belief
    unbelievable; unlikely; unconvincing.
  3. adjective satellite too improbable to admit of belief
    marvellous; tall; marvelous.
    • a tall story

WordNet

Im*prob"a*ble adjective
Etymology
L. improbabilis; pref. im- not + probabilis probable: cf. F. improbable. See Probable.
Definitions
  1. 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.

Webster 1913