let the cat out of the bag

1. To disclose a secret: “The mayor's visit was to be kept strictly confidential, but someone must have let the cat out of the bag, because the airport was swarming with reporters.”

Idioms and Phrases

Give away a secret, as in Mom let the cat out of the bag and told us Karen was engaged . This expression alludes to the dishonest practice of a merchant substituting a worthless cat for a valuable pig, which is discovered only when the buyer gets home and opens the bag. [Mid-1700s] Also see pig in a poke .

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