Let the cat out of the bag

One of the most curious and funny aspects of language is the use of idioms. These expressions use words in a non-literal way to mean something other than the actual meaning of the individual words—and they’re often quirky, funny, or perplexing, like “spill the beans” and “break a leg.”

Well, first, let’s consider the first recorded use of the phrase. In a 1760 book review in the London Magazine, the reviewer complained that he “wished that the author had not let the cat out of the bag,” presumably referring to some kind of plot point.

But while the first recorded use of the phrase is pretty straightforward, its origin is not. Unfortunately, it’s hard to pin down a precise origin for many idioms like these, simply because of the way language evolves. People start using phrases gradually, over time, and there’s no concrete historical record of how they came up with that phrase. This is especially the case with figurative expressions, which, by definition, don’t literally mean what they say.

The most heavily cited explanation for why we say “let the cat out of the bag” dates back to the Middle Ages. As the story goes, shady livestock vendors in medieval marketplaces sought to swindle their buyers. When someone would purchase a pig, the vendor would sneak a cat into the bag instead, cheating the buyer out of the higher price for a pig. It wasn’t until the buyer arrived home and, literally, let the cat out of the bag that they’d realize they’d been scammed, hence the phrase’s association with revealing a secret. English has no shortage of idioms for telling a secret—here’s why we also call it “spilling the beans.” This explanation is not outright proven—and neither, in fact, is the tale that medieval vendors even regularly did such a switcheroo.

But this explanation does have some things going for it. For one thing, per Phrases.org, both the Dutch and German versions of this phrase translate to “to buy a cat in a bag,” which alludes more directly to a deceitful purchase. And the Spanish translation means “to give a cat for a hare,” suggesting (somewhat more plausibly) that it would be rabbits, not pigs, that vendors would switch with the cats.

But, then again, the expression “pig in a poke,” which dates back to the 16th century, also refers to making a deal or purchasing something without fully validating it. (And a “poke” is an old-timey word for a bag.) For more idiom curiosities, why do we say “knock on wood“?

Today I Found Out theorizes that it might just be a far simpler explanation: just how stinking hard it would be to get a cat that was let out of a bag back into said bag. They theorize that this was nothing more than an artsy use of figurative language, comparing a now-widely-known secret to a loosed cat, rather than any reference to cats actually being in bags.

rd.com