When the liquid that comes from inside a vagina dries up and begins to flake off. It is often found on a man's penis after he has had his way with a lady.
Jeremiah: Yo how was that slut's snatch?
Jimmy: Wet as fuck, see for your self brah. *pulls down pants*
Jeremiah: Yo shit bro, that's a lot of cunt rust
on your cock!
As I fired my stream of piss into the toilet I looked down at my dick. To my surprise there was still cunt rust left over from last night's sexual shenanigans.