The term "asshat" became popular a few years back. Basically it was a snarky hip way of saying "stupid asshole" but at the same time getting around any potential censorship since "asshat" wasn't on any lists of proscribed words. But if you think about the term for any amount of time - it doesn't make any sense.
A hat is an item of clothing used to cover up the head. An item of clothing used to cover up the ass would most likely be a pair of pants or some shorts. Calling someone a pair of pants instead of asshat wouldn't be as snarky and would make no logical sense at all even if you are Brick Tamland from Anchorman. Maybe it's time to just go back to calling a jerk a stupid asshole.