The are called Hiphuggers because of a scene they had in season 1 (or so I'm told). If you look at the 5 pictures at the top, the one in the middle is from the scene.

They are called Hiphuggers because in one of their scenes in Season 1 they had a fight or something.. so when they made up they did this weird hug thing were they put there hands on each others hips :)

Maddy kinda already explained it ;)