Where does the phrase “pig in a poke” come from, and what does it mean?
The “poke” refers to a burlap bag or sack, which shoppers used to take to market centuries ago. At farmers’ markets in 14th century Europe, vendors were not uniformly honest. Unscrupulous merchants would surreptitiously substitute a stray cat of an appropriate weight for the suckling pig in the poke handed to an unsuspecting customer. Thus originated the warning: “Don’t buy a pig in a poke!” (“Don’t buy something sight unseen or without knowing its true value.” Today, the phrase has been extended to mean, “Don’t accept an unfamiliar object or idea on blind faith.”)
Another phrase originated from the practice of substituting a cat for a pig in a poke. It came from the moment the buyer discovers his true purchase. The phrase is … “D0n’t let the cat out of the bag!”
(Information from “The Word Detective” by Evan Morris. )