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"
The ultimate insult to someone who is more than just a cunt, or has just done something completely twatty. A clever use of the Scottish 'Mc' between 'cunty' and 'cuntlips' to make it sound like someones name, but also gives it a humourous feel, perfect for a bit of banter amongst friends.
Lame_d00d: I PwN3d J00 d00d!!!1111 D7: You Cunty McCuntlips ;-)