Let the cat out of the bag : This idiom means to disclose a secret suddenly. Imagine only you know the date of engagement fixed by another friend of yours. He has requested you not to disclose the matter to anyone else. However, if you tell this to your other friends in a gathering in front of who is going to be engaged, you have let the cat out of the bag, much to the embarrassment of your friend.
For example: Rani left the cat out of the bag when she told me about her boy friend.
Raj didn't like his friend's smoking habit. His friend's mother didn't know about it. Raj told his friends mother about this. Thereby he has let the cat out of the bag.