To masturbate. Comes from beat it up, which means to have sex, and jill, which means your hand.
Gina: "Damn I'm a total badass. I've never lost a fight."
Kim: "Bullshit. When the last time you kicked someone's ass."
Gina: "Well I beat up jill just yesterday behind the gym after math class."
Kim: "Whoa, too much information."