Sometime when you go to the movies you get that Coca Cola slideshow where you might get a film production vocabulary word (like "gaffer") with the definition.
Well I heard one on set that Coke will never show you.
See when you shoot two people talking to each other you usually shoot a wide shot.
And then you cover the conversation for each actor.
Now when you shoot someone for their close up, you can shoot them clean.
But it usually looks better if it's a little dirty. You keep a little piece of the other guys shoulder in the foreground, which creates a better sense of... space?
Usually the finished scene will incorporate a mix of both dirty and clean shots.
So sometimes a director will have the camera operator "keep it dirty" or "dirty it up."
But the other day because one actor was standing while another was seated.
And the director let the camera guy know that he wouldn't mind a little more "ass dirt" on it.
Ass dirt - n. A piece of out of focus rear end to be used in the foreground of an "over the shoulder" shot.