cat will be out of the bag, the

(slang) a secret will have been 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 returned home, he was incredibly surprised when he literally let the cat out of the bag