let the cat out of the bag

To let the cat out of the bag is to reveal a secret. But where does the phrase come from? What is the cat doing in the bag and what has this to do with secrets?

The phrase is a reference to an old scam in which a cat would be surreptitiously substituted for a suckling pig that had just been purchased at market. The cat would be placed in the bag in the hopes that the customer would not look into it until they were some distance away.

The phrase dates to at least 1760, although the scam itself is much older, dating to the 16th century at least. From London Magazine of 1760:

We could have wished that the author...had not let the cat out of the bag.

Also related is the phrase to buy a pig in a poke, which is a reference to the same scam (a poke is a bag or sack). And there is a similar phrase in French, vider le sac, literally meaning to empty the sack and used to mean to tell the whole story or finish the tale.

It’s commonly asserted that let the cat out of the bag refers to the cat o’ nine-tails used on board ships as form of punishment. The whip would be kept in a special bag to protect it from the sea air and to let the cat out of the bag was to confess a crime worthy of flogging. A neat tale, except there is absolutely no evidence to connect the phrase with a nautical origin.

(Source: Oxford English Dictionary, 2nd Edition)

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