The act of masturbating at either your or a coworker's desk in your office. It is preferrable to have a witness but the honor system can also be used.
John: Man I'm feeling better now.
Ted: Oh did you Desk Jack?
John: Yeah but not at my desk...I was over at Lynn's Desk.
Ted: Did you use an assist?
John: Yeah. I used that picture of her on vacation at the beach as an assist.