Refers to a joke which becomes much less funny if it requires an explanation.
Farcically derived from a nonsense situation whereby Einstein and his assistant at the time have just finished explaining, in great depth, the workings of hie great Theory of Relativity. Upon finishing the lecture, the gathered scientists and philosophers look a little non-plussed an tentatively applaud Einstein for his great work . . . though really they don't quite understand it. Whilst accepting the applause graciously, Einstein turns to his assistant, who happens to be chuckling, and mutters "See, I told you, it's nowhere near as funny when you have to explain it (!)"
First credit for this original word/concept must be given to Ian James Hay, for it was his peculiar mind that created it :)
British word-play and some puns lose they're humour when they're translated and/or explained to a foreigner; therefore they're einsteinisms.