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.