A female who masturbates. ie. (slapping her cunt). Could be considered the female counterpart to a wanker
. Its even more humorous to call your friend a "cuntslapper" than a "wanker" because it also implies that he's a female, and has a cunt that he enjoys slapping.
"hey joe, you cuntslapper! get in the car before we leave you behind!"
Person 1: "we had the week off of school, and the coaches still made us come to practice the whole week"
Person 2: "aww....fuckin cuntslappers"