Phantom masturbation as you see a lot of people use nowadays. The participant acts as if they are masturbating and sometimes concludes with a fake ejaculation. It is usually used as sarcasm or a way to show displeasure. The word is a play on the boxing technique of shadowboxing.
Extended version of muskedunder:
A privileged way to identify an enormous individual in terms of bodyfat. So large that the individual casts a large shadow over everything in its proximity.
Some other dude to his friend:
"Damn! It got really dark so quickly. Is it an ongoing solar eclipse at the moment?"
"Nah man! Look. It's gotta be that Shadowcasting Muskedunder rolling by."