Synonyms for Improbable:

adj.

all (adjective)
unlikely, unconvincing, unbelievable.
improbable (adjective)
incredible, unlikely, dubitable, implausible, farfetched, doubtful.
not likely (adjective)
unlikely, unbelievable, implausible, unsubstantial, questionable, unconvincing, weak, doubtful, inconceivable.

n.

long odds, there is no question of something, I don't suppose, dim, remote, you'll be lucky, not to be expected. weak, Unconceivable, shaky, unsubstantial, thin, inconceivable. improbable (noun)
unbelievable, supposed, incredible, unconvincing, marvelous, tall, unlikely, implausible, marvellous.

Other synonyms:

Unconceivable. Unapt. inconceivable, unreal. dim. questionable, unsubstantial. shaky. thin. weak. unlikely
remote.
Other relevant words:
unreal, Unapt, remote, strange, marvellous, unbelievable, shaky, unsubstantial, funny, weird, Unconceivable, questionable, tall, incongruous, weak, inconceivable, supposed, unconvincing, marvelous, thin.

Usage examples for improbable

  1. Satisfied that Grandall would not leave the clubhouse before morning, confident of his own ability to awaken at the slightest sound of footsteps near, and resolving to be astir before daybreak, anyway, if he were not disturbed earlier, which he regarded as quite improbable the scowling wretch allowed his eyes to close. – The Auto Boys' Mystery by James A. Braden
  2. It is highly improbable at that stage of the cause that he should have anything to urge which has not been already considered, but the ancient English practice in this respect is still followed, for it is not absolutely impossible that something may have occurred since the verdict that would affect the judgment. – The American Judiciary by Simeon E. Baldwin, LLD