Muck is something dirty or messy that spoils a place. When you walk into a muddy field after the rain, it's often covered in muck.
The word 'muck' can also be used as a verb. If you're working with heavy machinery and it gets stuck in the mud, you might need to use a shovel to muck it out. This means you'll be scraping away the dirt so the machine can move again. When people talk about politics or government, they often complain that things are being mucked up – made more complicated than they need to be.
Slimy mud, sludge.
Example: The car was covered in muck from the rally race.
