describes a person (typically female) that has obvious mental retardation combined with very attractive physical assets, such that the person is yet hotter because of the retardation -- likely because they are very easy to take advantage of
reference -- "Drawn Together", "The Other Cousin" (season one episode 5)
Captain Hero (about Clara's retarded cousin Bleh): Damn! Clara's cousin is so hot! nipples harden
Spanky: Yeah, she's, like, retarded hot!