A moist sticky mixture, especially of mud and filth. 2. Moist farmyard dung; manure. 3. Dark fertile soil containing decaying vegetable matter. 4. Something filthy or. Muck definition: Muck is dirt or some other unpleasant substance. | Meaning, pronunciation, translations and examples. muck meaning, definition, what is muck: mud, dirt, or a sticky natural substance such as animal waste. Learn more.