Basically, an Outdated Outfit
Maybe it's from a cool period of history far enough back it avoids passe fashion trends, but not so ancient it's out of place in the modern world. Perhaps it's the man or woman who makes the clothes and not the other way around, and something about their bearing makes what ought to be a desperately out of place Halloween costume work.
Usually from the victorian age, but regional chic from bygone eras can also work. Frequently, a significant portion
of their wardrobe is from the same era, but mixing it up isn't impossible.