I’d tell you an airplane joke...
I’d tell you an airplane joke.
But it’d go over your head.
This is an insult, and also a pun. To go over someone's head is a colloquialism that means to be too difficult for someone to understand, so this is saying that you are too dumb to understand the airplane joke that the jokester is about to make.
If you were to take the joke literally, then "to go over someone's head" could mean that this airplane joke (because it is an airplane) would travel in a path that was above the listeners head.