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)]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "improbable":
   absurd, crazy, distant, doubtable, doubtful, dubious, dubitable,
   extraordinary, fanciful, farfetched, fishy, flimsy, forced,
   hard to believe, hardly possible, implausible, impossible,
   inconceivable, incredible, insane, ludicrous, mad,
   more than doubtful, more than expected, out-of-the-way,
   past expectation, peculiar, questionable, quite another thing,
   remote, ridiculous, something else again, strained, strange,
   sudden, thin, unannounced, unanticipated, unbelievable,
   unconvincing, undivined, unexpected, unforeseeable, unforeseen,
   unguessed, unheralded, unhoped for, unlikely, unpredictable,
   unpredicted, unprepared for, unpromising, unrealistic,
   unsubstantial, unthinkable, weak, weird, wild, without warning