Ass Windex?
Well, they say the eyes are the window to the soul. So it seems there is no other logical explanation than that Mace's ass is an eye, which is a window to . . . well, something. (The soul of the Force?) Presumably not a very clean window if it needs Windex to be able to see into it.
I challenge anyone to try to disprove this, because you know you can't. Yeah, I said it.