adjective. ['ˌɪmˈprɑːbəbəl'] not likely to be true or to occur or to have occurred.
adjective. ['ˌɪmˈprɑːbəbəl'] too improbable to admit of belief.
adjective. ['ˌɪmˈprɑːbəbəl'] having a probability too low to inspire belief.