Adjectives

(a)improbable, unlikelynot likely to be true or to occur or to have occurred“legislation on the question is highly unlikely”, “an improbable event”
(s)improbable, unbelievable, unconvincing, unlikelyhaving a probability too low to inspire belief
(s)improbable, marvelous, marvellous, talltoo improbable to admit of belief“a tall story”