improbable
     adj 1: not likely to be true or to occur or to have occurred;
            "legislation on the question is highly unlikely"; "an
            improbable event" [syn: {unlikely}] [ant: {probable}]
     2: having a probability to low to inspire belief [syn: {unbelievable},
         {unconvincing}, {unlikely}]
     3: too improbable to admit of belief; "a tall story" [syn: {marvelous},
         {marvellous}, {tall(a)}]
Source: WordNet® 2.0