Pig in a Poke
(A). A blind bargain. The French say Acheter chat en poche. The reference is to a common trick in days gone by of substituting a cat for a sucking-pig, and trying to palm it off on greenhorns. If anyone heedlessly bought the article without examination he bought a “cat” for a “pig;” but if he opened the sack he “let the cat out of the bag,” and the trick was disclosed. The French chat en poche refers to the fact, while our proverb regards the trick. Pocket is diminutive of poke.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894