It's almost as if they have an invisible hat that says, "I'm an ass!". Hence, asshat.
(Plural: Asshats: A group of uncontrollable asses, usually young, immature boys, who are being complete jerks to everyone.
(Plural)-Tom and his buddies are being real asshats today.
B: You asshat.
Said persons' frequent attempts at humor usually lead to he/she making an ass out of his/herself. Not to be confused with ass-hole.
"Hey asshat, thanks for double parking."