don't let the cat out of the bag

The phrase "don't let the cat out of the bag" means to not reveal a secret or disclose confidential information to others. It suggests being cautious and keeping something hidden or unknown to maintain surprise, privacy, or an advantage. The phrase originates from a time when people would sell piglets in bags, sometimes substituting cats instead. If someone were to open the bag before the sale was complete, the swap would be discovered, hence the importance of keeping the cat (the secret) hidden.