Making a lowing sound. Bellowing. It’s from the Latin mugire, meaning “to bellow or moo.” Is “moo,” then, not merely onomatopoeic but also a shortening of this Latin word? Or is the original Latin itself onomatopoeic?
From the story “Actors” in the collection Dictation by Cynthia Ozick, page 79.
The usage in context here is pretty undistinguished, as the word appears in a list of words that the author of crossword puzzles happens to know. It’s not really worth quoting.