WordWeb Online

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