A muon is a type of particle like an electron, or a part of an atom. Basically, it's a physics term. The joke is similar to one that I know where it's an interrupting cow instead of a physicist. I'm not sure if you are familiar with knock knock jokes but the basic structure of the joke is: person A: knock knock person B: who's there? person A: <someone or something> person B: <someone or something> who? person A: <someone or something><joke>. The joke is usually a pun or play on words. In the interrupting physicist/cow joke, person A says "Muon!" or "Moo!" before person B gets to "who". The physicist or the cow interrupts person B. So that's the joke. Hope that makes sense.

Knock-knock. Leonard:Who's there? Sheldon: Interrupting physicist. Leonard:Interrupting physicist-- Sheldon:Muon!" The joke is not about a muon. The joke is that Sheldon is the interrupting physicist. He interrupts Leonard before he can finish "Interrupting physicist......."

It might be Moron! Or a version of the word Moron. It is an old word for dummy.

I googled the word "muon" Here's a partial definition: "The muon is an elementary particle similar to the electron,". Also, I think it's a "play-on" of words, similar of calling a person a moron. That's my opinion.