I learned that the cat you let out of the bag is the same animal as the pig in a poke
Nope, the cat is the cat o' nine tails, when it was taken out of its bag, it was because someone was about to be punished. No idea what he means by this comment.
Nope, the cat is the cat o' nine tails, when it was taken out of its bag, it was because someone was about to be punished. No idea what he means by this comment.
http://en.wikipedia.org/wiki/Letting_the_cat_out_of_the_bag
Note that "letting the cat out of the bag" is used in cases where the truth comes out, not when someone is about to be punished. Consider the following two sentences.
After the jury returned a guilty verdict, the judge let the cat out of the bag that the defendant would serve fifteen years.
At the conference, the Microsoft employee let the cat out of the bag that that Windows 9 wouldn't run on x86.
I would argue that the second feels far more natural than the first. That's because it's about uncovering the truth, like the fact that the pig in your poke is really just a cat, and not about punishment, like pulling out the cat o' nine tails.
Additionally, I'd point to the "pig in a poke wiki" page.
http://en.wikipedia.org/wiki/Pig-in-a-poke
The page shows that variants of this idiom are present in hundreds of languages, often with the statement that a person bought a cat instead of a pig or, occasionally, as rabbit. In fact, more culture refer to buying a cat in a sack than a pig.
The wiki page mentions your cat in the bag theory as well. However, if you follow the references, you'll considered that it is seen as dubious and that the pig in a poke explanation is considered far more likely.
Usually this is described in relation to a secret or a hidden truth: when it's out, you can't kill the truth, you have to live with the release of the information.
I have previously encountered references to swinging cats for entertainment or medical purposes, but not with credible sources.