this is where the rimmer is subjected to scat either from an anus that's dirty on the outside, or is clean on the outside but has a turd inside the anal passage, or where the top works some scat down from higher in the anal passage to the rimming area after the rimmer has throughly lubricated the asshole with spit and slobber. a dirty rim doesn't include eating scat, but can include shitting in the rimmer's mouth. the rimmer will then spit it out and continue the rimjob, until the ass is empty or the top has achieve an orgasm. The top will usually shoot on the chest, by may decide to lift up and shoot on the face or in the mouth of the rimmer.
I gave him a dirty rim and ended up shit-faced.