Mucky bum is derived from the ol' salty eye but it actually happens when you've tried to push out a fart that wasn't really a fart afterall thus creating the mucky ooze that starts the ol' salty eye process.
Hey man, I think I've got the ol' Mucky bum happening and I've gotta go finish up the paperwork.
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Mucky bum is the result of passing copious amounts of excrement and having it stick to unusually long anal hair.
I have a bit of a mucky bum this morning dude.