att köpa grisen i säcken

Literally: To buy the pig in the sack

Example: “Köp inte bilen utan provkörning, då köper du grisen i säcken.” — Don't buy the car without a test drive, or you'll be buying a pig in a poke.

English equivalent: To buy a pig in a poke

Origin: A Swedish adaptation of the German die Katze im Sack kaufen (to buy the cat in the sack), itself tied to a folk tale about Till Eulenspiegel, the trickster figure of German and Dutch folklore, who reportedly once sold a cat to an unsuspecting buyer by passing it off as a hare. The underlying practice being satirized was genuine: piglets, chickens, and other small livestock really were commonly sold at market pre-bagged in a sealed sack for easier transport, and a buyer who didn't insist on actually looking inside before paying risked discovering, only once safely back home, that a cat or some other far less valuable animal had been substituted for the piglet they thought they were buying. The closely related expression släppa katten ur säcken (letting the cat out of the bag) — revealing a previously hidden truth — comes from the exact same historical practice and period.