What does 'let the cat out of the bag' mean?

What it means: To accidentally reveal a secret or surprise that was supposed to be kept hidden.

"He let the cat out of the bag about the surprise party."
"I've let the cat out of the bag — pretend you don't know."

Where it comes from: Medieval markets, where unscrupulous traders would put a cat in a bag and sell it as a piglet. If the buyer opened the bag, the deception was revealed — the "cat was out of the bag." Also the origin of "buying a pig in a poke."

How it differs from "spill the beans": Both involve revealing secrets. "Spill the beans" is often deliberate or accidental. "Let the cat out of the bag" implies the secret was being hidden in something (a plan, a surprise) and someone opened it.

Register: Casual. Very common in everyday speech.

Tags: idioms, secrets, accidental, everyday English