endo-, 内,里面。-gamy, 结婚,词源同exogamy,bigamy.
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]