말이 (안)되다 (ma-ri (an)-dwi-da) – (im)possible/(un)believable
If you watch Korean dramas often, you would have surely heard the characters in the drama saying, “말도 안돼!” (impossible!) or “말이 되냐?” (can you believe it?/is it possible?/does it make sense?).
There is another phrase that is quite similar in meaning ([Vocabulary] 이럴 수가) but you would use 이럴 수가 in situations where you want to make an exclamation.
Another thing is 말도 안돼 is stronger than 이럴 수가 and have a negative connotation.
Last but not least, 이럴 수가 is mostly used in statements and not questions, which 말이 되냐 works extremely well.
Careful, though. These are informal expressions that we won’t usually use around people that we’re not close or intimate with.
와, 이 사람은 혼자서 산지 10년이나 된다고? 말이(도) 되냐?
Wow, you’re saying this person lived alone for 10 odd years? Is that (even) possible?
말도 안돼, 그 선생님은 40살만이라고? 20대 보이는데.
Impossible, you’re saying that teacher is 40 years old? She looks like she’s in her twenties.
아까 선생님은 내 성적을 밝혔는데 만점 받다고 하셔, 말이(도) 되냐?
Teacher revealed my results just now and said that I received full marks, is that (even) believable?