Let the cat out of the bag

Origin

The phrase likely stems from a common market swindle practiced in 18th-century England. Dishonest vendors would sometimes try to pass off a common cat as a more valuable suckling pig, often sold to unsuspecting buyers in a tied sack, known then as a "poke." A wary buyer, upon suspicion, might insist on inspecting their purchase, literally asking to "let the cat out of the bag." If the scam was uncovered, the deceit would be immediately exposed, revealing the true, less valuable contents. This vivid image of fraud and discovery quickly became a popular idiom for accidentally revealing a secret.