Old English har "hoary, gray, venerable, old," the connecting notion being gray hair, from Proto-Germanic *haira (source also of Old Norse harr "gray-haired, old," Old Saxon, Old High German her "distinguished, noble, glorious," German hehr), from PIE *kei- (2), source of color adjectives (see hue (n.1)). German also uses the word as a title of respect, in Herr. Of frost, it is recorded in Old English, perhaps expressing the resemblance of the white feathers of frost to an old man's beard. Used as an attribute of boundary stones in Anglo-Saxon, perhaps in reference to being gray with lichens, hence its appearance in place-names.
Old English forst, frost "frost, a freezing, frozen precipitation, extreme cold," from Proto-Germanic *frustaz "frost" (source also of Old Saxon, Old Frisian, Old High German frost, Middle Dutch and Dutch vorst), from the verb *freusanan "to freeze" (source of Old English freosan "to freeze"), from suffixed form of PIE *preus- "to freeze; burn" (see freeze (v.)). Both forms of the word were common in English till late 15c.; the triumph of frost may be due to its similarity to the forms in other Germanic languages. A black frost (late 14c.) is one which kills plants (turns them black) but is not accompanied by visible frozen dew.
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Definitions of hoarfrost from WordNet
ice crystals forming a white deposit (especially on objects outside);