dirt, rubbish, or waste matter Meaning, pronunciation, example sentences, and more from Oxford Dictionaries. Muck definition, moist farmyard dung, decaying vegetable matter, etc.; manure. See more. Define muck: wet dirt or mud — muck in a sentence. Encyclopedia article about muck. Sign up now Log in. See Lord MuckLady Muck. Irregardless It is in fact a real word but that doesn't mean you should use it. Ask the Editors How Often Is 'Biweekly'? Phrases Related to MUCK.