Let the cat out of the bag
This expression dates from the 18th century and means to disclose secret or confidential information, often by mistake. The origin remains unresolved but theories abound. It is often attributed to the removal of the cat o’ nine tails from the bag in which this fearful instrument of punishment was kept. The trouble with this theory is that it is not a good fit with the context. Naval floggings were public events designed to deter other members of the crew. There was nothing secretive about them, and there was nothing secretive about taking the cat o’ nine tails out of its bag. One supposes that a sailor caught committing some misdemeanour or other might say, "That means the cat's out the bag" but it is a bit of a stretch. The cat o'nine tails and its bag has little relevance to disclosing secret or confidential meaning. A second theory is the fraudulent practice in marketplaces of vendors selling someone a live cat instead of a live piglet, similar to buying 'a pig in a poke'(poke being an archaic English word for a pocket or bag). One must never buy a pig in a poke without looking into the bag otherwise one is inviting a swindle. Getting back to letting the cat out of the bag, one supposes that when one looks into the bag and sees that one has bought a cat instead of a piglet, one supposedly lets the cat out of the bag, thus revealing the secret. This theory about the origin is weak and even more tenuous than the cat o' nine tails theory, which is why the origin still remains unresolved. Will Rogers the American actor and comedian once said, “Letting the cat outta the bag, is a whole lot easier’n getting it back in.” he is absolutely right as anyone who has tried to put a cat into a bag would know. See also Sold a pup and Pig in a poke.