The cat's out of the bag

Where did the saying - "The cat's out of the bag" come from?

The most common explanation dates back to the Middle Ages. There were shady livestock vendors in medieval marketplaces.  When someone would buy a pig, the vendor would sneak a cat into the bag instead, cheating the buyer out of the higher price for a pig. It wasn’t until the buyer arrived home and, literally, let the cat out of the bag that they would realize that they had been scammed, hence the phrase’s association with revealing a secret. 

I really hope that they kept the cat as a furry soulmate!