Let the Cat Out of the Bag
When somebody is said to have let the cat out of the bag, they have revealed a secret, but – my oh my – ain’t that an unusual phrase?
So what, exactly, does feline freedom from sack imprisonment have to do with secrets being shared? If you have ever wondered about the origins of the phrase “Let the cat out of the bag”, read on and all will be revealed.
The generally accepted origins of the phrase date back to medieval England when piglets were sold in open marketplaces. To make it easier for the buyer to take the piglet home, the seller would hand it over in a sack. However, unscrupulous sellers would attempt to trick buyers by doing the ol’ switcheroo and instead of giving them the pig they had paid for, they would supply them with a bag o’ cat instead.
When a shrewd shopper would look in the bag and discover that he was being conned, out would jump kitty and the deception was revealed. The cat was literally out of the bag.
Source: Wikipedia
…where we get the phrase ‘buying a pig in a poke’ from, the poke being another word for a bag, and this expression meaning committing an unwise act!