Acronym for High School Whore. Basically, it's girls that wear slutty clothes and cheap-looking makeup, who are the slightly older versions of sesame streetwalkers.
Girl #1: Did you see that slut in fifth period?
Girl #2: Yeah. Ewwww. I heard she gave a bj last night at a party.
Girl #1: What a fucking HSW.
Girl #2: Deff.