Originates from "The Shit" meaning the best, and "It Girl" meaning the hottest female in a certain community (e.g. high school, or a fraternity). Put the two together and you have the ultimate chick, a Shit Girl.
Man, Laurie's the Shit Girl in mah house!
A jock who is also an ass.
Captain of the ______ team, AND a manwhore? What a jockass!