Quite simply: muck. A thick, soiled or filthy slop. Sometimes indicates any type of mess, tangle or jumble.
"High above the mucky muck
Castle made of clouds
There sits Wonderboy
Not much to say
When you're high above the mucky muck... Yeaaauh! Yeuh!
What is the secret of your power?
Won't you take me far away from the mucky-muck, man?"
-- Tenacious D
"NIGEL: This one...this 'course is a custom three-pickup-'Paul. This is my radio...unit....
MARTY: Oh, I see....
NIGEL: So I strap this...this piece on, you know, right down in here when I'm on stage and....
MARTY: It's a wireless.
NIGEL: Wireless, exactly. And...uh I can play without all the mucky muck.
MARTY: You can run anywhere on stage with that."
-- This Is Spinal Tap