Someone blurts out the details of a surprise party, and everyone groans: you let the cat out of the bag! It's one of our most familiar sayings. But stop and think about it for a second. Why would anyone have a cat in a bag at all?
The honest answer is that nobody knows for sure where the phrase came from. But one leading explanation is a scam from the old country market. A seller would claim to have a young pig in a sack, but slip a less valuable cat inside instead. The buyer wouldn't discover the trick until the bag was opened and the cat got out.
The Pig in a Poke
This theory is tied to another old saying: never buy a pig in a poke. A poke is an old word for a sack or bag. It comes from French and is related to the words pocket and pouch. Pigs were once carried to market in cloth bags, so buying one without looking inside was a real risk. The word poke is even still used in some regions today.
The phrase pig in a poke is recorded as early as 1555. Starting in the 1800s, people explained it with the story of a farmer swapping a cat for a suckling pig. When the buyer opened the sack, the cat was literally out of the bag, and so was the secret.
Many other European languages have sayings about this very trick. The French say acheter chat en poche, and Germans say die Katze im Sack kaufen. Both mean to buy a cat in a bag.
The pig in a poke has stayed popular with writers too. In his 1986 book Cadillac Desert, about water in the American West, Marc Reisner gave it a memorable twist when describing a dubious water project: the agency "wasn't even offering the farmers a pig in a poke; it was offering them a poke without a pig."
The Navy Whip Theory
There's a darker possibility too. One suggestion links the phrase to the cat o' nine tails, a whip once used as a punishment on Royal Navy ships. It was supposedly kept in a red sack. A sailor who reported another's wrongdoing would be letting the cat out of the bag.
This version makes a good story, but like the market theory, it hasn't been proven. The true origin of the phrase is still unclear.
Older Than You Might Think
The expression may be centuries old. According to a 2016 biography of Martin Luther by historian Lyndal Roper, Johannes Agricola referred to letting the cat out of the bag in a letter to Luther dated May 4, 1530.
Today the saying covers all sorts of reveals. It can mean exposing a plan to the person it's aimed at, explaining an inside joke, or giving away a movie's plot twist. Most often, it means revealing a secret by accident.
Around the World
Other languages have their own colorful versions of the pig in a poke idea. In Arabic, there's a saying about buying a fish in water. In Chinese, there's one about buying a cow on another mountain. In Spanish and Portuguese, people talk about buying a cat instead of a hare.
So the next time someone spoils a surprise, you can gently point out that they've continued a tradition that may be hundreds of years old. Just be glad it was only a secret and not an actual cat.
