The ninja jack. The act of jacking off silently (quickly in most cases) and without those around you knowing that you had even done so.
Usually refers to jacking off while someone is talking on the phone (I.e. a girlfriend calls and you rub one out while she talks about something random).
Girl: "Hey, so anyways my day blah blah blah"
Guy: "yup, sounds good"
Girl: "So- what do you think?"
Guy: "Sounds like a plan. We meet at your parents house for dinner tomorrow night"