Default term for when you need to reference an unknown, undetermined, or wildly irregular fetish. To be used as a placeholder and/or euphemism for said fetish.
Also known used in the long form "punching a bag of cats"
Jane: To have a good relationship you need to be open about everything that you're into.
Tom: But what if you're into punching a bag of cats?
Jane: Well that's something your partner would need to consider, isn't it?
When a woman doesn't wear any underwear, especially if her pussy is somehow exposed while not wearing underwear. You cannot call it going Commando like when a man doesn't wear underwear. It's called that because your dick is somewhat exposed to the elements.
"Dude, when the wind just blew a minute ago this hotties dress went up high and she wasn't wearing any underwear...Cat's out of the Bag!!"