S/he let the cat out of the bag.
->무심코 비밀을 누설하다.

[예문]
Sue:I just heard about my party tonight from John.
수:존한테 오늘 파티에 대해 들었어.
Bob:It was supposed to be a surprise party for you.
밥:너를 위한 깜짝 파티였는데.
Sue:Well, I guess he let the cat out of the bag.
수:글쎄, 그가 무심코 비밀을 누설한 거 같아.
Bob:I was hoping he could keep this secret from you until the party.
밥:파티까지는 비밀을 지켜주길 바랐는데.
Sue:Don't worry. I am not going to tell anyone I know.
수:걱정 마. 내가 알고 있다고 아무에게도 말 안 할게.
Bob:Thanks, I am sure others will appreciate it if you don't tell them.
밥:고마워, 네가 말하지 않으면 다들 고마워할 거야.