let the cat out of the bag

meaning:

To “let the cat out of the bag” means to tell information that was supposed to remain secret.

example sentences.

1. Maj Miller didn’t want to tell everyone about his new assignment until it was official, but he did tell his family. His children soon let the cat out of the bag by telling all of their friends.

2. Alice hasn’t told her best friend about her decision to marry Allan yet. She is afraid her friend will let the cat out of the bag before she can tell everyone the news herself.