A word for temperature, temperament, and approval
English speakers still use cool for a moderate chill: air that is not warm, yet not the bite of frost. The same form describes a person who stays composed, a palette of blues and greens rather than reds, a round sum of money named for emphasis, and, in casual speech, something stylish, impressive, or simply acceptable. The last cluster sounds like recent slang. The adjective underneath it is native English and older than the Norman Conquest.
The origin of the word cool is a Germanic inheritance, not a loan from French or Latin.
Old English already had the adjective and the verb
Old English wrote the adjective as cōl. It meant moderately cold, and it already reached into temperament: a person could be unperturbed, not heated by passion. Middle English kept the word as cole or cool; the modern spelling is a later regularisation of that native line. Historical dictionaries place the earliest English use in the Old English period, before 1150.
The verb is as old as the adjective. Old English cōlian meant to lose warmth and, already figuratively, to lose ardor. Around 1300 writers were using the same idea for intensity that dies down. A transitive sense, to make something less warm, is recorded from the late fourteenth century. A noun for a moderate state of cold appears around 1400. English therefore inherited a small family — adjective, verb, later noun — rather than a single isolated form.
Sister languages show the same verbal pattern: Dutch koelen, German kühlen, Old Saxon kolon. Another Old English verb, cēlan, meaning to make cold, survived in dialect as keel. Later English partly reshaped that verb so it would look more like the adjective cool. That is analogical leveling inside the language, not a second etymology.
The root it shares with cold
Historical linguists reconstruct a Proto-West Germanic adjective *kōl(ī) and a Proto-Germanic *kōluz. Handbooks differ slightly on how they mark the vowel (*kōluz in some, *koluz in others), but they agree on the family. That adjective sits on the same Germanic stock as the verb *kalaną, to be cold or to freeze. Behind that lies a Proto-Indo-European root usually given as *gel-, covering cold and freezing.
Cold is the closest English relative. It is not a synonym that arrived from elsewhere; it is an ablaut partner, a vowel-alternation sibling, of the base that also yields Old English calan. Cool and cold look like a pair because they are a pair.
West Germanic cognates are easy to hear: Dutch koel, German kühl, Old High German chuoli or kuoli, Middle Dutch coel, Middle Low German kȫl, West Frisian koel, Low German köhl. Old Norse kala means to be cold. Latin gelū, frost or chill, is cited as an Indo-European cousin, not as a word English borrowed. Slavic forms such as Russian холод, Polish chłód, and Czech chlad are often placed in the same distant family.
None of this is Latin-style prefix plus root. Cool is a simple inherited adjective. Later English derivatives — cooler, cooling, coolness, uncool — are ordinary affixation on that base.
How the senses multiplied
The temperature meaning never dropped away. What multiplied were figurative uses that treat heat as feeling.
By the early fourteenth century, cool could signal emotional coldness: apathy or dislike. A cool reception still uses that shade. From 1728 the adjective was attached to sums of money to stress the size of the amount: a cool hundred, later a cool million. Calmly audacious is dated to 1825. Cool-headed appears in 1742. To cool one's heels, to wait, often while kept waiting at a door, is attested from the 1630s; the usual explanation is resting hot feet after a walk.
The slang history is later and better known. Written English has cool as fashionable by 1933, first in African American vernacular. As a general term of approval it spreads from the late 1940s, through jazz and bop speech. Tenor saxophonist Lester Young is often credited with popularizing it in jazz circles; that credit is traditional rather than a tightly documented first use. Standard American dictionaries describe the mid-century positive slang as a development of an African American vernacular sense meaning excellent, first recorded in writing in the early 1930s and then carried by jazz musicians. That slang has stayed current far longer than most fashion adjectives that did the same job.
A separate scholarly argument connects the African American ethos of cool — composure, spiritual balance — to West African ideas, including Yoruba itutu, and to a contrast between cool and hot rhythms in drumming. That is a claim about culture, not a claim that the English word was borrowed from an African language. The lexeme remains Germanic. What happened in the twentieth century is a sense extension inside English, centered in African American usage and jazz, on top of an older figurative meaning already present in Old English: calm rather than heated.
The noun for self-control, the composure a person keeps or loses, is dated to 1966. Phrases such as play it cool sit on that figurative calm.
German still names the weather
German kühl is still, first of all, a temperature word. English cool never stopped being one either. What is unusual is that the Old English adjective for moderate cold also became a durable slang of approval, while rival slang of the same century mostly faded. The weather sense and the jazz sense look like two words to a casual reader. Historically they are one native adjective whose figurative life kept expanding, from an unheated temper in Old English to a twentieth-century yes that has not worn out.