A ruse, swindle, or suspicious transaction. In English, the more common phrase is "(to buy a) pig in the poke" (a "poke" being an older word for a bag or sack), meaning to buy something without verifying its contents or value first; the "cat in the sack" (a phrase more common to other European languages) refers to an item of lesser quality or value that has been substituted in its place. This is also the basis for the phrase "the cat's out of the bag" (and iterations thereof), meaning the swindle or secret has been exposed. I thought I was getting a great deal buying my car from that online seller, but as soon as I drove it home, I realized I'd bought a cat in the sack.