When a person jams their finger in their butt hole to prevent diarrhea from spilling into their pants
-Dude, is that guy scratching his ass in class?
-Naw man, that guy's doing a Thumb Plumb
A thumb plumb is when there is a lump of shit left on the thumb after thumbing an arse.
Holy shit, you should have seen the size of the thumb plumb she left.