A location frequently used for one's own masturbation, but also a word-play on the similarly sounding 'master bedroom' or 'master bathroom'. This can be effective jab or pun that the owner of the house jacks it or whacks it alot.
Jack: "...And finally here at the end of the hall, are the bedrooms."
Me: "Oh, and might I assume this one is the masturbatorium?" ::smiling::
Jack: "Wha--Excuse me, what did you say?" ::agitated::
Me: "May I assume this one is the master bedroom?" ::speaking slowly, for clarity::
Jack: "Why, yes it is." ::relieved::
Me: "Well, I can imagine you spend alot of time in there." ::smiling::