1. addict (from an addict's habit of scratching self)
2. sound made when manipulating a record back and forth in a rhythmmatic fashion
3. windows "scratched" by graffiti writers

Tattoos, especially old school prison tats.

to engage in a sexual act with, most commonly intercourse

In track, when a runner withdraws from a race before it begins. The term originated from the days when heat sheets were written on paper, and a runner's name would litterally be scratched off in pencil when he/she decided not to race.

cash money!!

Extramarital sex, extracurricular sex.

Verb: To 69, or conduct oral sex with your partner while they simultaneously reciprocate.

In a more common context, the word "scratch" is applied to the activity of rasping, or digging with the fingers, or nails. Examples of this useage can be seen in the phrase, "You scratch my back, I'll scratch yours." In this slang context, that phrase is amended to, "You suck my junk, I'll lick yours." Of course, those verbs can be further amended to allow for gay/lesbian scratching, or different oral techniques.

Verb: The self-stimulation of a female's external reproductive organs, especially through repeated back-and-forth motions. This word is most accurately applied when the female is getting a "quick fix", and masturbating through her undergarments. "Scratch" is both a description of the motions involved with, and sound produced by this word's proper execution.