‘Let the cat out of the bag’
When someone has let the cat out of the bag, it means that they have revealed information to someone else that the latter did not previously know. The phrase’s origins are not entirely clear, but there are two possibilities.
The first is that it is a reference to the cat-o’-nine-tails whip, which was used as a punishment on Royal Navy ships — a sailor informing on another sailor’s disobedience would be said to have “let the cat out of the bag.”
The second common explanation refers to a common scam in which someone buying a suckling pig would actually be sold a cat instead, but would not realize this until they opened the bag they were given.
