Cats love to explore. They crawl under the couch or jump on the windowsill to watch the world go by. Leave a box on the floor and a cat will almost certainly make itself cozy. You can also find cats inside bags, at least metaphorically. “Letting the cat out of the bag” implies revealing a secret, whether by mistake or intentionally. But why “cats and bags,” of all the combinations? Why not “dogs and cages” or “mice and shoeboxes”? Unfortunately, we can’t be certain of this phrase’s origin story, but we can make an informed guess.
One popular theory relates to the idiom “buying a pig in a poke” (“poke” being a type of burlap bag). The phrase dates to the mid-16th century and is understood as buying something without prior inspection. When this saying came about, people were sometimes sold bags purported to contain piglets, but they actually contained cats (which were cheaper and more plentiful). This fraud may have given rise to the idea of cats secretly being stuffed into bags — and the need to let them out of the bags.
Another theory has to do with a cat-o’-nine-tails — a multi-tailed whip used to flog undisciplined sailors, sometimes stored in a bag. Researcher Gary Martin, founder of the site Phrase Finder, says this one has a reasonable timeline on its side, as the idiom “let the cat out of the bag” was documented in 1760, about 65 years after the whip was being used on the decks of the British navy ships.
This early printed example of the phrase comes from a 1760 book review in The London Magazine. The review says, “We could have wished that the author … had not let the cat out of the bag,” likely talking about a criticism of the plot being spoiled. The context suggests “cat out of the bag” was already popular and would be known by the readers by then.
However, Martin says this and the previous theory are a bit dubious, so we’re left to draw connections where we can. We can only offer up these theories on the phrase’s origin. Unfortunately, we couldn’t let the cat out of the bag even if we wanted to, as the exact origin may be lost to time.
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