The act of pacing the room while masturbating, often characterized by bumping into furniture the closer you get to climax. Usually done late at night by insomniacs and results in a high amount of random bruising.
"Every night at like 3am we can hear his floorboards squeaking as he walks around. He must be masturpacing."
Josh: Where's Phil been lately?
Bob: I don't know man, I think he's masturdating again.
Josh: I thought he got over Handgelina Jolie?
Bob: He did, it's Lindsey Lohand know.