This has two means to it actually...
1. When your kitten plays in a pit full of rusty metal scrap pieces.
2. The act of rubbing a rusty pipe around on your vagina.
1. Dude you clean kitten is going to turn into a rusty kitten.
2. Honey? Why did you give yourself a rusty kitten??
The butt, or more specifically, anus
-So how's your new girlfriend
-She's ok, but she won't take it in the rusty kitten