Urine Idiot
The more literal translation of "you're an Idiot" it's shorter to write as a sentence and both sides get a good laugh out of the situation.
Douche: Hey Man, I have feelings too, just because I bang 20 chicks a day doesn't mean I'm a douche..
Funny Guy: Man, You're an Idiot. Get it? Urine Idiot! HAHAHA!
Douche: HAHAHAHAHA!
Funny Guy: Man, You're an Idiot. Get it? Urine Idiot! HAHAHA!
Douche: HAHAHAHAHA!