“Let the cat out of the bag” means to reveal a secret, usually by accident or before the right time. People use it when someone tells information that was supposed to stay hidden.
A widely repeated origin story connects the idiom to dishonest market selling. In that story, a seller promised a buyer a piglet in a bag, but secretly put a cat inside instead. When the bag was opened, the cat jumped out and the trick was exposed. That clear image helps explain the modern meaning: a hidden truth suddenly becomes known.
The exact history is not completely certain, but this explanation is common because the link is easy to understand.
- Example: I let the cat out of the bag about the surprise party.
- Example: He let the cat out of the bag when he mentioned the new job before the announcement.
This idiom is informal and very common in everyday English. It often appears with secrets, surprises, plans, gifts, and announcements. If you use it, the idea is not just that information became known, but that it was revealed earlier than it should have been.