late 14c., Scottish, "heathen, pagan," and having that sense first in English; as an adj. from late 15c. from Latin ethnicus, Greek ethnikos, from ethnos "band of people living together, nation, people," properly "people of one's own kind," from PIE *swedh-no-, suffixed form of root *s(w)e- (see idiom).
In Septuagint, Greek ta ethne translates Hebrew goyim, plural of goy "nation," especially of non-Israelites, hence "Gentile nation" (see goy). Sense of "peculiar to a race or nation" is attested from 1851, a return to the word's original meaning; that of "different cultural groups" is 1935; and that of "racial, cultural or national minority group" is American English 1945; ethnic cleansing is attested from 1991.