improbable in WordNet English dictionary

1. not likely to be true or to occur or to have occurred
   "legislation on the question is highly unlikely"; "an improbable event"
   improbable  unlikely 
   
2. having a probability too low to inspire belief
   improbable  unbelievable  unconvincing  unlikely 
   
3. too improbable to admit of belief
   "a tall story"
   improbable  marvelous  marvellous  tall