Masturfarting, as opposed to its also popular brother activity, Farturbating, is when one engages in the act of masturbating while they pass gass. The order of these activities is crucial : If you are gassy (i.e. farting) and you elect to masturbate, you are masturfarting. Congratulations.
For disambiguation, see FARTURBATING, and the sister activity, QUEEFURBATING
Ron: "Why does it smell so weird in here?"
Dave: "Sorry, Ron, I was masturfarting. That should explain the smell, and the residue on your sheets."