The name is buttless, not assless for a very simple reason. All people have an ass, but not everybody has a butt. In order to be the proud owner of a butt ones ass must possess a shape other than flat.
I had a butt at one time and it was an incredible butt. Unfortunately I lost it in a nasty & bitter divorce.
The only reason I tell you about this is because many people do not know the difference between an ass and a butt. On many occassions I have been told it can't be done when I tell somebody to kiss my ass.
Basically it was a toss up between buttless and flatass.