What's an Asshat?

The name "asshat" people sometimes call each other keeps popping up. Today, I asked myself, what is an asshat? I know its supposed to be a disparagement, I think I've used it myself. But it really doesn't make sense. Is it supposed to be that the person is actually a hat you wear on your ass? Is it that the person's ass is a hat, and other people put it on? Is it that the person likes to wear other people's asses as hats? Shouldn't the insult be then "asshat wearer"?

I'm so confused. This page, "Asshat: The Official Site", while amusing, doesn't enlighten. Neither does the Wikipedia entry.

Update:
This entry from the Urban Dictionary explains it most cleary:

One who has their head up their ass. Thus wearing their ass as a hat. Asshat

Posted September 8, 2005 11:51 AM