When you scratch your ass and you end up with poop under your nails. This can also be achieved if your finger pokes through the toilet paper while wiping.
Damn bro, my ass was itchy when I went to bed last night and I woke up with rusty nails.
By Lynnell
Rusty Nail
When you are having sex with a girl on a beach.
Then you pull out cover your dick with sand and ram it back into her vagina
It will feel like you rammed a rusty nail up inside her