Retardfucker
Someone who is fucking/ having sex with a retard. Not necessarily someone with mental retardation though just someone they've name-called a retard.
Guy: You're a retard!
Girl: If i'm a retard then what does that make you?
Guy: A retardfucker :D
Girl: If i'm a retard then what does that make you?
Guy: A retardfucker :D