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......."

Simplest answer. the reason the joke is funny is because it parallels an old children's joke about the interrupting cow. in this joke, the joke teller will interrupt by saying MOOOO! In the Big bang theory joke, the physicist also interrupts by saying MUUUUUON! which is both a thing in physics and also a mirroring sound to MOOO.

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.