A euphemism for the verb form of that most pleasurable and accessible of activities. In written form it is innevitably the concluding word and is followed by ellipses. When spoken, it again finishes the sentence, but with the addition of trailing off so as to emphasize the implications of the phrase.
Morgan: I was bored last night after school so I locked my door...
Tom: Do you mean masturbated?!
Morgan: My mom says that's a dirty word.
Tom: Grow some hairy man testicles and call it what it is!