Etymology

I like the phrase, "a pig in a poke," partly because of the image it brings up (a poke is a bag or sack: it's dropped from the language, but we still use the diminutive, "pocket"). So, you're buying a wriggling sack, but the piglet is only assumed: the phrase itself refers to someone buying something sight unseen.
Why does this phrase enter the language? Because sometimes people would take advantage of others, and place another animal of dubious economic value in it's place: the cat. (I stress "economic value" for the cat-lovers among my readers, because I'm also fond of cats. But you can't eat 'em). So, buying a "pig in a poke" is to take a risk on something sight unseen.
One reason I find this particularly interesting is that this is also the origin of another phrase: "let the cat out of the bag." (How often was this going on, anyway? Were there lots of gullible people routinely buying piglets in sacks?) You've given away the secret: it isn't really a pig at all. The cat runs away, and you're left holding the bag.
Actually, that's a phrase that's been running through my head lately, but it seems to have a different origin. They can't all be about buying piglets, after all.
You'd think the gap since I last posted would mean that I'd have something really good today. Sorry.