Pig-in-a-poke is an idiom that refers to a confidence trick originating in the Late Middle Ages, when meat was scarce but apparently rats and cats were not.
The scheme entailed the sale of a "suckling pig" in a "poke" (bag). The wriggling bag actually contained a cat—not particularly prized as a source of meat—that was sold unopened to the victim.
A common colloquial expression in the English language, to "buy a pig in a poke," is to make a risky purchase without inspecting the item beforehand. The phrase can also be applied to accepting an idea or plan without a full understanding of its basis. Similar expressions exist in other languages, most of them meaning to buy a cat in a bag, with some exceptions:
