Why is the ninth month called September?
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.
Although the term 'ethnic cleansing' has come into English usage only recently, its verbal correlates in Czech, French, German, and Polish go back much further. [Jerry Z. Muller, "Us and Them: The Enduring Power of Ethnic Nationalism," Foreign Affairs, March/April 2008]