Answer: "That's not a big deal." 난 / 또 / 뭐라고 I / again / (thought) something's happening 완전 똑같은 어조로 말하자면, "Whelp, that's not a big deal!" In the very same nuance, "Whelp, that's not a big deal!" 보통은 타인이 자기에게 무슨 말을 하려고 해서 기대했다가 사소한 거라는 걸 알았을 때 자주 쓰여요. They usually say this when someone was going to tell them something, they were expecting and it turned out to be trivial. 불안해했다가 안심하거나, 기대했다가 실망하거나, 뭔가 대수롭지 않다는 걸 알았을 때 말해 보세요. "난 또 뭐라고!" When you're relieved after being nervous or disappointed after expecting, or find something trifling, just say: "난 또 뭐라고!"

난 또 뭐라고 = 난 또 뭔가 큰일이라고 생각했네 I thought it also was a something serious.