Fecal matter residue left near the opening and linings of the anus that was not properly wiped off and has thus solidified. These poo particles can remain for days and are often only expelled through showering or through farting which causes these particles to explode in small clouds of dust, leading to the term "poo dust."
(Friend to roomate) I just farted and I'm pretty sure the last remnants of my poo dust have now evaporated.
The dust-like faecal matter/resiude that ends up on the toilet seat when someone careless wipes their butt without due care and attention. Shows up worst of all on white toilet seats. And when someone wipes standing up.
Babe, when you leave the toilet please can you check to see you haven't left any poo dust or skidders?!