Pig in a poke

This idiom usually appears in the form of an injunction 'don't buy a pig in a poke'. Poke is an Old English word from the 13th century for a small bag or sack. It is linked, etymologically, to words like pocket, pouch, poach, and poacher, a person who puts ill-gotten gains into a bag or sack. The French poche meaning pocket derives from the same etymology. So 'don't buy a pig in a poke' is a caveat emptor urging unsuspecting buyers to beware of fraud and take a look in the bag to see if it really is a pig (it's more likely to be a piglet because, of course, a fully grown pig is unlikely to fit into a small bag). Unsuspecting buyers might find they bought a pup or a cat instead of a piglet, which was a commonplace fraud, practised at street markets since the Middle Ages. Thus, pig in a poke is linked to expressions like 'bought a pup' and 'let the cat out of the bag'. The advice not to buy a pig in a poke is first recorded in John Heywood Proverbs (1546) but it was probably used verbally long before this. See also Bought/sold a pup and Let the cat out of the bag.