A pig in a poke is a thing that is bought without first being inspected, and thus of unknown authenticity or quality. The idiom is attested in 1555:

A "poke" is a bag, so the image is of a concealed item being sold.

Starting in the 19th century, this idiom was explained as a confidence trick where a farmer would substitute a cat for a suckling pig when bringing it to market. When the buyer discovered the deception, he was said to "let the cat out of the bag", that is, to learn of something unfortunate prematurely, hence the expression "letting the cat out of the bag", meaning to reveal that which is secret. The French idiom acheter (un) chat en poche and the Dutch een kat in de zak kopen and also the German die Katze im Sack kaufen (all: to buy a cat in a bag) refer to an actual scam of this nature, as do many other European equivalents, while the English expression refers to the appearance of the trick. The English idiom "sold a pup" refers to a similar con.

In common law, buyers have the right to inspect goods before purchase.