mooncalf |ˈmoōnˌkaf|noun ( pl. -calves )a foolish person.ORIGIN mid 16th cent.: from moon + calf, perhaps on the pattern of German Mondkalb. Originally in the sense [shapeless mass in the womb,] thought to be produced by the influence of the moon.
I came across this word while looking for alternatives for the word "idiot" in my 1961 edition of Rodale's Synonym Finder. I love this book; it's so much more than a thesaurus.
In the end I didn't use mooncalf in my MS, but it's an interesting word to remember. Although, I can't imagine many occasions when I'll have reason to use it.