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 ;-)