pig in a poke

Something that is purchased without having been thoroughly inspected, often with negative consequences. A "poke" is a bag. Purchasing a home without inspecting it first is like buying a pig in a poke.

An object offered in a manner that conceals its true value, especially its lack of value. For example, Eric believes that buying a used car is buying a pig in a poke. This expression alludes to the practice of substituting a worthless object, such as a cat, for the costly suckling pig a customer has bought and wrapping it in a poke, or sack. It dates from a time when buyers of groceries relied on a weekly farmers' market and, unless they were cautious enough to check the poke's contents, would not discover the skullduggery until they got home. The word poke dates from the 13th century but is now used mainly in the southern United States. The idiom was first recorded in John Heywood's 1562 collection of proverbs. Also see let the cat out of the bag.

Something that is offered in a manner that conceals its true nature or value.

An item bought without prior inspection. A poke is a “bag.” Purchasing something that you've not seen is the basis of the image—you don't know the condition of the pig in the bag until after you've bought it. It may be exactly what was advertised, or it may be something much worse than what the seller described—the swine!