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2.
The after effects of inserting the penis into the dirty ass of another. When the penis is pulled out the tip of the cock is lightly covered with poo thus giving it the look of having a mud on the helmet.
Man I was hittin her in the ass and when I pulled out I had a mud helmet.
by Mr. Mud Helmet June 03, 2009
 
1.
The fecal matter caked to one's penis after sexual relations in another's anus.
Dude, after ramming Alice in the ass, I had a serious mud helmet!!!
by cobramark20 December 27, 2008