shibboleth
Sad to say I didn't find it funny at all. Not the first time. Not this time.
Reminds me of when my friends all thought that Vic Reeves and Bob Mortimer were the most hilarious thing on TV and I tried and tried to get the joke but just had to give up in the end.
I think it's a deep rooted thing, like being able to make a tube with your tongue or speak the right dialect of a "click" language. You got it or you ain't.
I ain't.