From Where Does The Phrase 'Let The Cat Out Of The Bag Originate'?

To let the cat out of the bag means to spill the beans or to let a secret out.  It is thought to have originated in the late middle ages.  Farmers used to carry their pigs to market in bags which were closed and then tied up to stop the pig escaping and to stop it kicking.
Some unscrupulous traders, when they were selling little piglets, would actually fill the bag with a cat instead.  
This trick would generally work, until someone asked to see the piglet.  If the bag only contained a cat, then the secret would be out and the con would fail as the 'cat came out of the bag'.
A similar expression 'straight from the horse's mouth'  also originates from markets.  Horse sellers would often declare a horse to be younger than it actually was, so the prospective buyer would look in its mouth to check its age and so get the truth 'straight from the horse's mouth'.