The most common rationalization for this term is "I am not a clown made of ass, how dare you accuse me of being such!" In all reality, the term "assclown" is much deeper than one would be lead to believe.
Assclown is possibly the worst insult you could give a person, and is in most cases synonymous with "failure at life."
"Oh, hey, yeah, you see that Russian guy? Yeah, he's an assclown."
"You're an assclown, Val."