To shotgun masturbate is to be able to jack off in one swift, shotgun pump-esque motion. This is due to the girl you are masturbating to being so ridiculously hot, that one shotgun pump motion is all you need.
Guy #1: Dude, how hot is she?
Guy #2: I swear, I could shotgun masturbate to her.
Guy #1: Really? I don't believe?
Guy #2: No lie. I swear. Just chk, chk, boom.