To let the cat out of the bag is an idiom, meaning to reveal a secret, and the best theory we have of its origin stems from a pig in a poke. Don’t panic, I’ll explain that one, too.
A pig in a poke is another idiom, meaning a product that is bought in packaging, thus the contents are not visible to the buyer upon purchase. The name itself derives from purchasing suckling pigs at markets, which were kept in bags or sacks – or a “poke” – to prevent them from escaping. “Poke” comes from the French “poque“, which is where we get “pouch” and “pocket”.
The theory being that people were sold such bags under the pretence that inside were juicy, delicious suckling pigs when, in fact, they were being sold an incredibly less valuable cat. The “letting the cat out of the bag” being the wholly too-late realisation of the customer that they had been scammed into buying said cat, and therefore the fraudster’s secret coming to light.
This is referred to by John Heywood in 1555, who wrote;
I wyll neuer bye the pyg in the poke Thers many a foule pyg in a feyre clokeJohn Heywood, 1555
There are translations in French, Dutch and German for this idiom, indicating that it was a common enough scam throughout European societies.
This version has faced much criticism, simply because it seems someone would have to be rather dense to mistake a cat for a piglet – the Spanish equivalent seems more plausible, “dar gato por liebre“, “giving a cat instead of a hare”.
The idiom itself has no record until 1760, however, and saw a surge in usage throughout the late 18th Century, which implies that it was a new-fangled thing.
Although we may not know for a certainty where the phrase came from, one thing is certain…
Letting the cat out of the bag is a whole lot easier than putting it back in.Will Rogers