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1.
(n). A feces latent anus sighting during the action of sex.
(n). A person's ass you might think to be rather dirty. A dirty ass.
I was having sex with this girl and when I flipped her over she had a rusty bumper.

I bet that girl has a rusty bumper.

by Andy and the Rad April 20, 2009
 
2.
When you're having sex, you shit in your hand and smear it across the buttocks of your partner, and proceed to hump aggressively.
David pulled a fucking rusty bumper on Max, and he didn't even see it coming!
by club210 May 07, 2009