When you don't wipe yourself well enough after defecation, and residual feces begins to smear itself between your cheeks.

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.

Pat Hutchison has the title of "Grease Cheeks due to his constant wet flatulence.