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A poop smeared ass crack. Usually the result of improper wiping or cleaning. May also be the result of a shart or fart in which you get more than you bargained for.
I started to go down on my girl last night until I discovered she had a nasty mud gutter. Then I got the hell out of there.

Or

Man, I shouldn't have pushed so hard with that last fart. Now I got a mud gutter!
by The Gronk March 02, 2010
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