"marriage within the tribe or group," 1865, from endo- on model of polygamy. Related: Endogamous (1865). Opposed to exogamy. Apparently both were coined by Scottish anthropologist John Ferguson McLennan (1827-1881) in "Primitive Marriage."
To this law, the converse of caste, forbidding marriage within the tribe, Mr. M'Lennan has given the name of exogamy: while, instead of caste, since that word involves notions unconnected with marriage, he has used the correlative word -- endogamy. [review in "The Lancet," March 25, 1865]