Inserting your penis, dildo, strap-on, fist, ect. inside someone's ass while they are pooping to push the poop back up their ass.
The man put his penis inside the girl's ass right as the poop started to come out, causing the poop to go back in her ass and causing them both to have a not so pleasant feeling but the great feeling of knowing that you accomplished it.

-he is mud plunging her, he mud plunged her, she plunged herself, I mud plunged that filthy whore-

(hint: it would make your life easier for you if you didn't tell people that you mud plunged someone)
by epg2 November 11, 2011
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When having sex and you accidentally ram it into her ass and pull out with shit stuck on your wang.
Jessica and I were banging last night and I accidentally mud plunged.
by Dandy lop May 5, 2016
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