(slang) a secret will be found out. The expression, cat's out of the bag goes back to the 8th century when it was illegal to sell and eat pork. Pork lovers had to purchase their pigs in bags under the cover of darkness. Frequently, dishonest farmers would place a cat in the bag instead of a pig. So when the unsuspecting pork lover returnedhome, he was incredibly surprised when he literally let the catout of the bag. —AbilityCongress Approved Glossary