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let the cat out of the bag

let the cat out of the bag idiom

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.

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.