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When you can't clean your anus after pooping no matter how many times you wipe. Usually a mechanical pencil shit is one that takes a long time to come out, so one begins to wipe prematurely before the whole poop has completely dislodged itself from the butt hole.

One's shit keeps refilling like a mechanical pencil: no matter how many times that one wipes, he can't get it clean.
This mechanical pencil shit took two rolls of toilet paper to wipe and I still haven't stopped shitting.

Mentioned by Adam Carolla on the Adam Carolla Podcast 8-11-11
by pzeke August 11, 2011
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