I was curious about the origin of the expression buying a pig in a poke.

It turns out that it dates from at least 1555:

I wyll neuer bye the pyg in the poke
Thers many a foule pyg in a feyre cloke

Or in modern English:

I will never buy the pig in the poke
There’s many a foul pig in a fair cloak

A poke is a sack, and pigs were brought to market in sacks or pokes and sold that way. If the buyer didn’t examine the contents of the poke before he bought it, he might regret the purchase. Hence, the expression to buy a pig in a poke means to regret a purchase or an action.

And why might the buyer regret the purchase? Well, sometimes a farmer might substitute a cat for a suckling pig. (You’ve got to keep a close watch on those tricky farmers.) The buyer discovered the substitution when he let the cat out of the bag. And we have the origin of a second phrase.

Two for the price of one.