To let the cat out of the bag is to reveal a secret by mistake. Do not let the cat out of the bag.
The mistake is the whole of it, since deliberate telling is not letting anything out. The old story about a cat sold as a piglet may or may not be true and is always repeated.
1phraselet the cat out of the bagTo reveal a secret by mistake.
“One question at the wrong moment let the cat out of the bag.”
let the cat out of the bag is a phrase.
The phrase appears in the eighteenth century and is usually explained by a market trick of selling a cat as a piglet.
First use appears c. 1753 or 1770. 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.
Etymology from Wiktionary, CC BY-SA
Practised in Idioms