White or gray, usually from age.
My father is fond of a kind of coughdrop-like candy called horehounds. I knew from his description that the candy got its distinct flavor from a plant of the same name, but what I did not know is the relation to the word hoary. When I opened my Chambers Dictionary today looking for flibbertigibbet, the book opened into the Hs and the entry for horehound caught my eye. According to Chambers, it is «a hoary labiate plant.» Hore is also listed as an alternate spelling of hoar, and hoarhound as an alternate spelling of horehound, so it stands to reason that the plant is so named for its characteristic hoariness.
I don't have any etymological citation for this relation, so don't quote me on it. In fact, there are varieties of horehound called both white horehound and black horehound (which are part of the same family but not the same genus) so there may well be some other origin to the name.
Hoary itself is a word that I've encountered for decades without ever bothering to find out what it meant.