A Lesbian. "Box" being the female sexual apparatus, and since penetration will not "naturally" occur with other lesbians (nothing natural about plastic), "bumping" describes it.
" rollin through the hood cold tearin shit up
stick my head out the window and i say was up
to the niggas on the corner cold bumpin the box, but you know thats an alabi for slangin the rocks". - eazy duz it written and composed by my favorite rapper eazy e. RIP eazy we love you man.