You're likely to come across "lanuginous" in only a few contexts, botany and spelling bees being the best candidates. In other contexts, the more common term is "downy." "Lanuginous" has an unsurprising pedigree. It's from the Latin word lanuginosus, which is in turn from "lanugo," the Latin word for "down." ("Lanugo" is also an English word used especially to refer to the soft woolly hair that covers the fetus of some mammals.) "Lanugo" itself is from lana, meaning "wool," a root also at work in "lanolin," the term for wool grease that's refined for use in ointments and cosmetics.