A Pig in a Poke

[tamarinne]

Last night I said, in the course of conversation, "I don't want to buy a pig in a poke."
Rick: "I've never heard anyone say that out loud before.  What exactly DOES that mean?  Poke the pig before you buy it, make sure it's still squealing?"
Me: "I don't think so... I think a poke is like a bag or a crate or something.  You shouldn't buy a pig that you can't see."

This led to wikipedia (To the Internetz, Batman!) and some really fascinating information (subject to the caveat that we found all this on Wikipedia, which could mean it is complete or partial fiction.)

Selling a pig in a poke (bag, from the French "poque".  A tiny bag, or "poque-ette", is the source of our word "pocket") was a medieval confidence scheme.  People would put cats in bags and pretend they were pigs, and sell them for meat.  There are versions of this phrase in almost every European language.  Most of them are versions of "don't buy a pig in a bag" or "buying a cat in a bag".  One interesting variation was the Romanian "being caught with the cat in the bag". 

It is believed that this is also the origin of the phrases "let the cat out of the bag" and "be left holding the bag".