A label for people, and for other people
In present-day English, ethnic usually points to a group bound by ancestry, language, religion, or custom. We speak of ethnic groups, ethnic identity, and ethnic minorities. In looser talk the same adjective often marks food, clothing, or music as belonging to a culture the speaker treats as other than the default. That second habit is newer than the first, and it is not how the word began.
The origin of the word ethnic is a long loop: a Greek adjective for what belongs to a people, a church Latin term for the outsider in faith, a late medieval English word for a heathen, and only much later a scholarly and then political term for cultural and national groups.
Borrowed through the church, not inherited
English did not receive this adjective from Old English. It is a borrowing. Late Latin ethnicus already carried an ecclesiastical load: Gentile, pagan, someone outside Christian (and, behind that, Jewish) worship. That Latin form restates Greek ethnikos. English took the word in the late Middle Ages, first as a noun for a heathen and soon as an adjective with the same religious sense. A close earlier relative, ethnical, is on record in the same period.
The first English date is not a single agreed year. The Oxford English Dictionary cites evidence before 1475 in a version of Ranulf Higden's Polychronicon. Other historical dictionaries put a noun around 1400 and the adjective in the later fifteenth century; Merriam-Webster's first dated adjective sense is 1542. The scatter is ordinary for a medieval loan: the word is in English by the fifteenth century, even if each glossary's earliest surviving line differs. Spellings such as ethnik and ethnick show the form had not yet settled.
Ethnos and the suffix that made an adjective
Greek ethnikos is built in a clear way. The noun ethnos takes the ordinary adjective ending -ikos, the same piece English later writes as -ic. Ethnos meant a company of people living together — a nation, a tribe, a people — and could also name a flock or swarm of animals. At that layer it is closer to a group of one's own kind than to a modern nation-state.
Classical writers did not use the adjective in only one way. Historians could mean national, or in keeping with a people's customs. Grammarians used it for names and habits that marked foreigners, including the demonym of a city. In the Roman Empire it could also mean provincial. Surface analysis in English as ethno- plus -ic is fair for the modern word, but the Greek formation is older than the English combining form.
Behind Greek, the usual reconstruction ties ethnos to a Proto-Indo-European reflexive root, *s(w)e-, which also yields words for one's own and for the in-group. Some etymologists give a suffixed form *swedh-no-. Another line of analysis routes the noun through a formation related to Greek ethō, to be accustomed, and some dictionaries note a kinship with ēthos, custom. The reconstructions are not identical in every detail. They do agree that the older idea is social belonging, not race in the later European sense.
Distant relatives of that reflexive root include Latin sodalis (companion), Sanskrit sva (one's own), and Gothic swes (one's own). Those are cousins of the root, not loans of the English adjective. Direct later cousins of ethnic itself include French ethnique, Italian etnico, Spanish étnico, German ethnisch, and Modern Greek ethnikós, which in ordinary Greek more often means national.
The combining form ethno- later built ethnography, ethnology, and ethnicity. Those learned compounds, coined for the study of peoples, almost certainly helped tug the older English adjective back toward a civic and cultural sense after centuries of religious use.
From Gentile to minority
The religious meaning English first adopted was not a medieval invention. In the Septuagint, the Greek translators of the Hebrew Bible used ta ethnē, the nations, for Hebrew goyim, a word that often meant nations other than Israel. Christian writers then used ethnikos for what was Gentile or pagan: alien to the worship they counted as true. That is the sense English kept for several hundred years. An ethnic, as a noun, was a heathen — someone not a Christian or a Jew.
The older Greek meaning did not stay buried. By the mid-nineteenth century, English had begun to restore a sense close to peculiar to a race or nation (one widely cited date is 1851). The new -graphy and -logy compounds gave that restoration a scholarly home. In the 1930s the adjective was being used of distinct cultural groups. By the 1940s, especially in American English, it named a racial, cultural, or national minority, and the noun returned in a new guise: an ethnic as a member of such a group. Dated evidence for that noun clusters around 1941 to 1945.
A further everyday sense — ethnic as a label for minority or foreign food, dress, and style — sits on that twentieth-century use. It treats an unmarked majority as the norm and everything else as ethnic, a habit the word's own path makes easy to notice. The grim compound ethnic cleansing is later still, attested in English from 1991, though parallel phrases had already circulated in other European languages.
The detour that still shows
If one older fact is worth keeping, it is the biblical detour. English did not first meet this adjective as a polite term for heritage. It met it as a church word for the outsider in religion, because Greek had already used the nations for peoples beyond Israel. The modern cultural sense is a return toward the classical meaning, not a straight line from Polybius into the census form. The reflexive root under ethnos still fits: a word about who counts as our kind, and who does not.