Region: UK | Usage example: I let the cat out of the bag about their engagement.

Let the cat out of the bag

Meaning

To reveal a secret, usually by accident or earlier than intended.

Used for spoiled surprises, leaked plans, or private information becoming public. The tone can be playful or critical depending on context.

Origin notes can vary by region and source; this page gives the commonly reported explanation and usage context.

Often explained through old market fraud stories where a seller substituted a cat for a piglet in a bag. The origin is plausible but not certain.

The phrase is associated with English rural and market imagery and became widely used for accidental disclosure by the 18th and 19th centuries.