A guitar effect housed in a small casing (usually made of metal) and it is operated by foot.
The guitarist from Incubus used a wide array of stompboxes in his live performance.
Girl who is always getting you down, can be a bitch, snob or just depressing. Can also be used as just a general term for girls that you want to insult. Also applicable to feminine emo males.
Man that girl is such a stompbox! or What a fucking stompbox!