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®

improbable
     adj 1: not likely to be true or to occur or to have occurred;
            "legislation on the question is highly unlikely"; "an
            improbable event" [syn: {unlikely}] [ant: {probable}]
     2: having a probability to low to inspire belief [syn: {unbelievable},
         {unconvincing}, {unlikely}]
     3: too improbable to admit of belief; "a tall story" [syn: {marvelous},
         {marvellous}, {tall(a)}]