if you watch Grey's Anatomy then you are familiar with McDreamy, McSteamy, and McVet.
Well, McSlut is a highschool name given to an attention whore girl who craves attention and gets it by dating 2 hideous guys at once and then denying it.
she claims they are all "just friends."
she claims to have emotional issues to get them to pay more attention to her.
This is what her 'friends'(people who put up with her, aka kaceyhaters; see kacey cutter
call her behind her back or around her so that she doesn't know who they are talking about.