Hoarfrost
Etymology
From "hoar" (Old English "hār," meaning "grey, aged, venerable") plus "frost." The frost’s white coating is imagined as the grey hair of age — the same "hoar" survives in "hoary."
noun · /HOR-frost/
A covering of delicate white ice crystals that forms directly from water vapor on cold, clear nights.
What it names
The intricate, feathery frost that seems to grow like fur — and hides an old word for "grey-haired" in its name.
Example
Hoarfrost feathered the window into a garden of white ferns.
Related idea
Hoarfrost forms from vapor turning straight to ice; rime forms when existing fog droplets freeze on contact.