'Improbable' definitions:

Definition of 'improbable'

From: WordNet
adjective
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]
adjective
Having a probability too low to inspire belief [syn: improbable, unbelievable, unconvincing, unlikely]
adjective
Too improbable to admit of belief; "a tall story" [syn: improbable, marvelous, marvellous, tall(a)]

Definition of 'Improbable'

From: GCIDE
  • 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]