A pig in a poke is a thing that is bought without first being inspected, and thus of unknown authenticity or quality.
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.
[5] 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.
[7][8] A poke is a sack or bag, from French poque, which is also the etymon of "pocket", "pouch", and "poach".
In the April 1929 edition of the literary magazine The London Aphrodite, a story by Rhys Davies, titled "A Pig in a Poke", was published, in which a Welsh coal miner takes a woman from London for his wife and regrets it.