Ever had someone accuse you of letting the cat out of the bag? Unless you do some very questionable things with cats, you probably haven’t actually let the cat out of the bag. But what’s the origin of this idiom?
One possible explanation is that the “cat” that’s being referred to is a whip instead of a feline. The Cat O’ Nine Tails was a whip used mainly in the Navy, especially during the Napoleonic Wars in the early 1800s. A Cat O’ Nine Tails, was a whip with nine different strands of rope. We think it was called Cat O’ Nine Tails because of the claw like marks it would have caused. “Cat out of the bag” could be connected with seeing the Cat O’ Nine Tails as a form of punishment, but we don’t know how this could be linked to disclosing a secret.
The more probable explanation is connected with markets, from circa 1500. Merchants would sell piglets in bags, known as a “pig in a poke”. Back then, “poke” would have meant a bag or sack, stemmed from the French word “poque” of the same meaning. “Pig in a poke” was essentially the same as “caveat emptor” (buyer beware).
If buying a pig in a poke, you would always need to check if the animal was indeed a piglet, as merchants had a reputation for replacing the valuable piglets for something practically worthless – feral cats. So, the putative theory is that this idiom comes from when you open a poke and release a rather annoyed feral cat, instead of a piglet.