N이었을/였을 줄이야.

It is attached to a noun when a certain event, an object, or a person of the past was unexpected because they were unknown to the speaker at that time. It’s used like a talk to oneself.

민수의 첫사랑이 나였을 줄이야.
I can’t believe Minsu’s first love was me.

세상에, 그 낡은 물건이 그렇게 값나가는 물건이었을 줄이야.
God, I can’t believe that old stuff was such a valuable thing.

저 사람이 그 사건의 범인이었을 줄이야. 꿈에도 생각하지 못했어요.
I can’t believe he was the culprit in the case. I never thought of it even in my dream.

Conversation:

이 사진 좀 봐. 남자 친구하고 데이트할 때 찍은 사진인데 잘 나왔지?
Look at this picture. I took it while on a date with my boyfriend, doesn’t it look good?
응, 근데 여기 덕수궁 돌담길 아냐? 여긴 연인이 같이 걸으면 헤어진다는 속설이 있어.
Yes, but isn’t it the stonewall street of Deoksu Palace? There’s a myth that if a couple takes a walk together there, they break up.
정말? 왜 그런 속설이 생겼어?
Really? Why there is such a myth?
확실치 않지만 이혼하려는 부부들이 그 길을 지나갔기 때문이래. 예전에는 근처에 가정법원이 있었거든.
I’m not sure, but people say it’s because couples who want to divorce have passed the street. There used to be a family court nearby.
맙소사, 이렇게 예쁜 길이 그런 장소였을 줄이야. 남자 친구랑 헤어지게 되면 어떡하지?
Oh, my god. I didn’t expect such a beautiful street to be a place like that. What should I do if I break up with my boyfriend?
너무 신경 쓰지 마. 그냥 속설일 뿐이야.
Do get too worried. It’s just a myth.

Related words
돌담 stonewall
속설 myth
이혼 divorce
가정법원 family court

Source: Yonsei University. Korean Language Institute