Hello my little kittlings,
Some knowledge requires prior initiation, some concepts are simply too complicated to illustrate in soundbyte chunks of processable conversation. Whenever you begin to explain an idea, and opinion, a fact, only to realise that it is really just a little bit more complicated to elaborate than it will be satisfying for your interlocutor to learn, you are dealing with a Pampelmousse. 
By the time you have finished your explanation, what should have been an “Oh-my-gods, that’s amazing” has petered down to an “Oh… right… okay,” and you both feel cheapened by the experience, you wish that you hadn’t gotten into it at all.
In its most proper use:
“We’re making some incredible advances in nanosatellite swarm algorithms, these days.”
“Oh really, what’s that?”
“It’s a bit of a Pampelmousse, to be honest, you had better look it up.”
 Yes, that’s the French word for a Grapefruit, misspelt. I could explain, but it’s something of a Pampelmousse, you see…