When I was at (UK)school in the fifties we were taught that in medieval times, market days were held & people bought piglets sewn into a bag called a 'poke'. Thus you were never to buy 'a pig in a poke' because it could have been a cat, hence (as mentioned above) the buyer could warn others about the swindler by 'letting the cat out of the bag'.

If not true, it shows you should never rely on teachers!