Buying a pig in a poke

At some time or other we've all been warned not to buy a 'pig in a poke'! But how did that saying originally come about?  To buy a pig in a poke means that you have been cheated. Back in Saxon times a suckling pig would be taken to market and sold in a sack, or poke. However, there were some shady characters around, and an unsuspecting buyer could find that a cat had been substituted for a pig in the poke.