Cold weather, cold manner
In present-day English, frosty still does the obvious job. It names air cold enough to leave ice on grass, a night that will freeze a puddle, a lawn or windshield filmed with crystals. A glass can look frosty when condensation or ice sugar-coats it. The same adjective also names a social climate. A welcome, a reply, or a meeting can be frosty when the people in it keep their distance. Nothing in the modern word requires a new root for that second use. English has long let weather vocabulary stand in for feeling.
The origin of the word frosty is native, not borrowed. Speakers did not import it from French or Latin in the age of print. They inherited an adjective that already belonged to English in the Old English period, written forstig and fyrstig, and used of winter cold. Standard historical dictionaries place that use before the twelfth century. An English rendering of Bede’s ecclesiastical history, from about the year 900, already applies fyrstig to a grim, freezing winter. The later spelling is newer. The adjective is not.
How the form settled
Middle English keeps frosty and the variant forsty. Both sit on the Old English line. In the living language the word is transparent: the noun frost plus the ordinary adjective-forming suffix now spelled -y. That reading matches the history. Old English -ig is the same suffix, the one that yields icy, dusty, and misty. It marks a quality of the noun, not a separate root.
Some desk dictionaries date the English adjective to the later fourteenth century. That note usually describes the Middle English spelling in their files, not the first time English had the word. Noun and adjective both passed through a stretch when two shapes competed. Old English had forst and frost for the noun; the adjective shows the same split. Metathesis moved the r: forst beside frost, forsti beside frosty. By the end of the fifteenth century the unmetathesized frost had become the usual noun in the standard language, in part because it lined up with cognate nouns elsewhere in Germanic. The adjective followed. Frosty, not forsty, is the form that survived.
The noun behind the adjective
Frost, the noun, is Old English frost and forst: frozen precipitation, a freeze, bitter cold. Behind it lies Proto-Germanic *frustaz, built on the verb that gives English freeze (Old English freosan). That verb family is carried further back to a Proto-Indo-European root often reconstructed as *preus-, with two senses that look at odds: to freeze and to burn. The pairing is not a riddle invented for English. Cold that sears and heat that sears were named with related material in more than one branch. Latin pruina, hoarfrost, is commonly treated as a distant relative of the Germanic frost words, not as the source of the English noun.
For the adjective, comparative work reconstructs Proto-West Germanic *frostag and *frustīg. Those starred forms are scholarly estimates of a stage no one wrote down. What the record actually shows is a frost noun plus an adjective suffix, inherited inside West Germanic rather than taken from outside it.
Neighboring languages did not need to lend English the word. Parallel formations, each built at home, include West Frisian froastich, Dutch vorstig, German frostig, Low German fröstig, and Swedish frostig. English frosty is a cousin of those adjectives, not a copy of any one of them.
From ice on the ground to ice in a greeting
The oldest English uses are meteorological and bodily. The word describes wintry weather, a freezing country, air so cold that breath can be seen, a person who feels the chill. Middle English also uses it, more rarely, of a beard that is white. That sense is visual. Frost on a field is pale. Hair or beard that looks dusted with rime can be called frosty without any claim about the temperature of the person wearing it. Later English still reaches for that picture when it wants hoary without saying old in so many words.
The figurative social reading is on record from the late fourteenth century. Cold already named a lack of feeling. Frosty made the lack more specific: a surface that will not thaw. Later glosses speak of cooled affection and of a manner without warmth or nerve. Present-day English keeps the reserved, unfriendly sense and has added everyday extensions that still rest on the same image, from a frosty reception to a frosty glass. The weather meaning never left. The human meaning layered on top of it.
There is no need for a folk story to explain the shift. The suffix did not wander in from French. The aloof sense did not wait for modern etiquette. Speakers used a weather word for human weather, as they did with cold, icy, and chill. When sources disagree, they disagree about dating labels, not about the path. Old English already had the adjective; Middle English respelt it; the polite chill is the later overlay.
The suffix, and what it is not
The -y of frosty is the native adjective suffix, historically -ig. It prefers short stems. Frost is a single stressed syllable, so frosty sits with the old weather set instead of sounding like a comic extra beat. Frostily and frostiness are later, regular derivations from the adjective. They are not separate borrowings and they do not change the etymology of the base.
Because the parts are still clear, it is easy to treat frosty as a modern transparent coinage. The written record runs the other way. English was already calling winter fyrstig when the language was still inflected, centuries before print.
One date worth keeping
Carry away the early winter citation, not a tour of every cognate. Around the year 900, English prose could already call a season fyrstig. A frosty welcome, in the sense of a chilled reception, is hundreds of years younger. Dutch vorstig and German frostig show that the North Sea languages kept matching adjectives of their own; they did not have to supply English with this one. Frosty is home-grown, old, and still able to freeze a field or a conversation with the same inherited stem.