Wiktionary
vb. (context transitive English) To clean the excrement and other rubbish from the area where an animal is kept, such as a horse stable or a dog kennel.
Usage examples of "muck out".
And don't you worry, I'll get to work at scraping the muck out, just like you said.
Hours of trotting the horses around and around a straw track in a frozen field, hours brushing the dust out of their coats, hours of cleaning the muck out of their boxes and carrying their water and hay, hours of lying awake at night listening to the stamp of the horses below and the snores and mumblings from the row of beds.
We'd do a damned sight better to start getting muck out and gettin' some timber ready.
He wondered about that all the next day as well, while shoveling smelly black muck out of the cistern.
She pulled the hoofpick out of her belt, and began cleaning the packed muck out of it with studious care.
Only when Zagorka flung the muck out of her eyes did she recognize the centaur.
That evening I learned how to muck out a horsebox, and the next morning I started to ride.
Phidestros looked again, then clawed muck out of his beard and grinned.