The origin of the idiom "let the cat out of the bag" is quite interesting and dates back to old market scams. In the past, piglets or small animals were sometimes sold in bags at markets. Dishonest sellers would sometimes substitute a less valuable animal, like a cat, for a piglet and sell it in a bag to an unsuspecting buyer.
If the buyer became suspicious and opened the bag before completing the transaction, the seller would have "let the cat out of the bag," revealing the trickery and dishonesty.
Over time, this literal expression evolved into a metaphorical one, meaning to reveal a secret or disclose information that was intended to be kept hidden. So, when someone says "let the cat out of the bag" today, they mean that a secret has been revealed or that confidential information has been disclosed.
