You use the phrase ‘Pig in a Poke’ to indicate that an offering or deal has been foolishly accepted without prior examination.
Example of use: “Be careful about buying that old car. It might turn out to be a pig in a poke.”
The origin of the expression ‘pig in a poke’ comes from the 1500s, when a “poke” was a sack, and merchants would sell piglets in pokes, often sight unseen. When an unsuspecting buyer got his poke home and went to release the piglet, a chicken, duck, goose, or some other animal less valuable than a pig would come out of the bundle instead. The advice being given is “don’t buy anything until you have seen it.”