When in the marines, you find a female creature (normally human) to have sex with. You bring them to a muddy space, dress both of you up in camo, but still exposing your genitals/or other fetishist need. Proceed to have sex in the mud.
James: Dude I had a sick night with Sheila!
Michael: Heh, that's why you're so muddy! You did a mucky camo din'cha.
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.