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To clean the place where an animal lives or is kept, especially by removing excrement, soiled materials, food waste, etc. A noun or pronoun can be used between "muck" and "out." I was given the unenviable task of mucking out the horses' stables when I spent the summer on the ranch. The pigpen gets absolutely putrid if you don't muck it out every day.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.