let the cat out of the bag
- (idiomatic) To disclose a secret; to let a secret be known, often inadvertently.
- It was going to be a surprise party until someone let the cat out of the bag.
- 1849, Charlotte Brontë, Shirley
- He proved himself as decent, decorous, and conscientious as Peter was rampant, boisterous, and — This last epithet I choose to suppress, because it would let the cat out of the bag.
to disclose a secret, often inadvertently