まさか! That can’t be true! / You don’t say so [tell me]! / 《口語》 No kidding!
きのう彼女に会ったよ.―まさか!―いや, 本当だよ. I saw her yesterday.―Oh, you’re kidding!―No, I really did.
あれで英語の先生だって? まさか. Him? An English teacher? You must be joking! / He is a teacher of English? Never!