When a person feels an itch in their butt crack and scrapes a couple of fingers across their asshole only to sniff them therefore confirming that they now have shit smelling fingers. Everybody in the world has done this or they are an outright liar!

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.

When your Lover lets you go down on her and you do a "Stink Check" which is you sticking you fingers in their ass then proceding to twirl around ending it by pullin them out then sniffing and though is a stink check