let the cat out of the bag

Cultural  

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.”

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let the cat out of the bag Idioms  

1. 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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