Tim Bowen sheds some light on the origins and definition of the phrase to let the cat out of the bag.

This means to reveal a secret, often when it is dangerous or embarrassing to do so, as in “Everything was OK until he opened his big mouth and let the cat out of the bag. Then we were all in big trouble”.

The expression appears to originate in medieval England when peasants farming the land were required to give a large part of their income to the lord of the manor. Some tried to avoid declaring all their animals and frequently took them off to sell them illegally at a market or at a butcher’s shop. Young pigs were often sold in this way and were carried in sacks to the market. As the sales were illegal on the part of both buyer and seller, they were often carried out at great speed and sometimes it was possible to sell a cat instead of a much more valuable pig because the buyer did not look in the bag before the sale. If they did, however, and the cat jumped out, the secret was revealed.