A white, crystaline frost that forms by the same process as dew on surfaces that have been cooled below freezing. Feathery forms occur when the surfaces are freezing before the dew condenses and a more blobby form occurs if the dew condenses as a liquid and then freezes.
From Old English har and later hoary meaning “white or grey with age” especially of hair, “venerable” and “old” in reference to the white colour of the frost.
In Titania’s speech from A Midsummer Night’s Dream, Shakespeare uses an intermediate simile, which prefigures modern use:
And thorough this distemperature we see
The seasons alter: hoary-headed frosts
Fall in the fresh lap of the crimson rose,
And on old Hiems’ thin and icy crown
An odorous chaplet of sweet summer buds
Is, as in mockery, set.