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Adjective: improbable  im'pró-bu-bul
  1. Having a probability too low to inspire belief
    - unbelievable, unconvincing, unlikely, doubtful
  2. Too improbable to admit of belief
    - marvelous [US], marvellous [Brit, Cdn], tall
  3. Not likely to be true or to occur or to have occurred
    "an improbable event";
    - unlikely

See also: implausible, incredible, supposed, unbelievable

Encyclopedia: Improbable