A racial term for an illegal Mexican immigrant. It originated from the myth that Mexicans crawl under the U.S fence to get into America. In the proccess, their backs get scratched from the fence. (Also, see "wet backer")
When a woman and a man are engaging in missionary style sex, the woman wraps her unshaven legs around the male's back and proceeds to rub up and down, thus resembling a back scratcher.
"Yeah dude, I was fuckin this chick last night and she totally pulled a chico back scratcher on me! I look like Jesus after he got whipped now!"