to let the cat out of the bag (to accidentally reveal a secret)
A: I’ve done something terrible!
B: What have you done?
A: I told Brian about the surprise birthday party we are going to have for him!
B: Oh no! You’ve really let the cat out of the bag, haven’t you!
A: Greg doesn’t know that you failed your driving test, does he?
B: Yes, he does. Helen let the cat out of the bag last night. She told him when she was drunk.
A: You can’t trust her with secrets. You shouldn’t have told her.