What does "let the cat out of the bag" mean — and why is it funny?

informal

Meaning

To reveal a secret, usually carelessly.

Where it comes from

One popular theory is a market scam where a piglet was sold in a bag but a worthless cat was hidden inside; opening the bag exposed the trick.

Why it is funny

It is funny because the secret becomes a live, uncooperative animal. Once a cat is out of a bag it is never going back in, and it will not sit still or stay quiet. The phrase captures how a leaked secret takes on a life of its own.

Used in a sentence

"He let the cat out of the bag about the new baby before they were ready to tell everyone."