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.

• "It was going to be a surprise party until someone let the cat out of the bag."
• "[H]e 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)."

• spill the beans
• talk
• tattle
• blab
• peach
• babble
• sing

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