Source : Webster's Revised Unabridged Dictionary (1913)

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.

         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.

Source : WordNet®

improbableness
     n : the quality of being improbable [syn: {improbability}] [ant:
          {probability}]