Muck is thick, wet, dirty material like mud mixed with rotting plant matter, manure, or general filth. You might find muck at the bottom of a pond, in an animal barn that hasn't been cleaned, or in a swamp where decaying leaves have turned the ground into a squishy mess that sucks at your boots.

The word carries a sense of disgust or unpleasantness. Clean garden soil isn't muck, but that grimy sludge in a clogged storm drain definitely is. When farmers talk about mucking out a horse stall, they mean shoveling out the dirty straw and manure, which is hard, smelly work that has to be done regularly.

People also use muck to describe a confusing mess or difficult situation: “I really mucked up that math problem” means you made a mess of it. When something gets mucked up, it becomes disorganized or ruined. British speakers sometimes say muck about or muck around to mean fooling around or wasting time instead of working.

The phrase where there's muck, there's brass (brass meaning money) suggests that dirty, unglamorous work often pays well, because someone has to do it even though it's unpleasant.