Wordorigins.org

Buy a pig in a poke is a confusing phrase to many, mainly because people aren’t quite sure what is meant by poke. Poke is a dialectal term for bag or sack; in the US, the word tends to be found in the Appalachian region. So to buy a pig in a poke is make a blind purchase, to buy something sight unseen. The word poke dates to c.1300 when it appears in The Lay of Havelok the Dane:

Hise pokes fulle of mele an korn.
(His pokes are full of meal and corn.)