Top Definition
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
by James Rox Sox January 28, 2009
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.
by Biggaybear April 02, 2013
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