let the cat out of the bag

(Mid-1700s) The inverse of the idiom pig in a poke. If a dishonest merchant tries to sell a cat as a pig and the cat comes out or is taken out of the bag, the merchant's secret is disclosed.[1][2]

let the cat out of the bag (third-person singular simple present lets the cat out of the bag, present participle letting the cat out of the bag, simple past and past participle let the cat out of the bag)

1. (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 let a secret be known): spill the beans, talk, tattle, blab, peach (obsolete), babble, sing

• pig in a poke
• buy a pup
• sell a pup
• be sold a pup
• get off one's chest
• bare one's breast