If the Cat Is Let Out Of The Bag some sort of secret has been made known. In medieval markets, ruses were often played on unsuspecting buyers. One involved piglets and cats. A person hoping to purchase a piglet from a trader would be shown a suckling piglet. Then, whilst he or she was haggling with the trader over the price, the piglet would be substituted for a cat, which would be bagged up. The buyer would only discover the deception upon arriving home and letting “a cat out of the bag”.