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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
(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