Let the Cat Out of the Bag (Origin)

What Is the Origin of the Saying "Let the Cat Out of the Bag"?

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The term "let the cat out of the bag" means to expose a secret.

Examples of Use:

Oops, I think I let the cat out of the bag about your pending promotion.
We will arrange a press conference. Do not let the cat out of the bag until then.
My sister told my parents that I was engaged. She deliberately let the cat out of the bag.
Sorry about the crowd outside. Somebody must have let the cat out of the bag about your visit.

The origin of this idiom is uncertain, but it could relate to the practice of market stall owners fraudulently selling a cat in a bag as opposed to a more valuable piglet in a bag. If the cat were let out the bag, then the secret would be revealed.

Detractors of this theory highlight that nobody would mistake a cat for a pig, adding that the practice would be far too risky for the stall owner. However, supporters of this theory note that similar sayings exist in other languages, e.g., "Een kat in de zak kopen" (Dutch) and "Die Katze im Sack kaufen" (German). These sayings both translate as "to buy a cat in bag," and they both mean to buy false goods. Supporters of the theory cite this as evidence that fraudulently passing off cats in bags as something else was a known practice.

My personal suspicion is that there is a story from a European country that involves a cat in a bag being passed off as something else.

Competing Theory

In this saying, the word "cat" refers to a cat o' nine tails, a nine-tailed whip used to punish sailors. The "cat" was kept in a red bag onboard the ship, and if any misdemeanour came to light (i.e., the secret was uncovered), then the "cat" would be taken out of the back.

The "cat o' nine tails" features in two other well-known sayings too: