In english, "I know you are, but what am I" is a childlish rebuttal to name-calling. It's like saying "no, it's not me, it's you" or "it's the one who says it who is".

It's the degree zero of rebuttal becaude there's no argument. In this joke, the rebuttal actually works in a court setting and seems to convince the judge.

Yeah, explain for us dumbasses