A Dirty Nikko is when you tell your girl close her eyes, then insert each testicle separately into the vagina and anus. After doing so, put each one in her mouth and have her guess where each one was.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.