Let The Cat Out of The Bag

 Why do we say let the cat out of the bag? This old saying is probably derived from the days when people who sold piglets in bags sometimes put a cat in the bag instead. If you let the cat out of the bag you exposed the trick. I have heard it said, on certain historical ships I could mention that it comes because a cat o'nine tails was kept in a bag. But the cat o'nine tails was not used by the English navy till the mid-17th century and the saying is certainly older than that.