The act of masterbation in the shower or bath.
Chris was showerbating, so it took him forever to get ready.
Process of masturbating while taking a shower. Very convenient, due to the fact that most often ejaculation can easily be washed down the drain, the running water makes the activity more pleasurable, and practically every substance around is a lubricant.
Roommate: God, why does it take so long for Jacob to shower?
Passer-by: Isn't it obvious, he probably does showerbation every chance he gets!
Roommate: Oh gross, I hope he washes all the jizz down the drain.