The Collaborative International Dictionary of English v.0.48:

Improbable \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.
   [1913 Webster]

         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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

improbableness
    n 1: the quality of being improbable; "impossibility should
         never be confused with improbability"; "the improbability
         of such rare coincidences" [syn: improbability,
         improbableness] [ant: probability]