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):

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: improbable, unlikely] [ant:
           likely, probable]
    2: having a probability too low to inspire belief [syn:
       improbable, unbelievable, unconvincing, unlikely]
    3: too improbable to admit of belief; "a tall story" [syn:
       improbable, marvelous, marvellous, tall(a)]