'Let the cat out of the bag' means accidentally revealing information that you weren't supposed to reveal.
Example of use: "Tim let the cat out of the bag about my surprise birthday party."
Most of us have let the cat out of the bag at some time. The origin of the idiom “let the cat out of the bag” is an interesting one. In medieval times, livestock such as piglets, chickens and ducks were bagged for sale. Some dishonest livestock brokers put cats into bags rather than the actual animals advertised for sale; the cats were about the same size and weight, but were considered to be of little value. When unsuspecting customers returned home and discovered the secret, they were said to have “let the cat out of the bag.”