1751, "adoption," from Fr.
affiliation, from M.L.
affiliationem (nom.
affiliatio), from L.
affiliatus, pp. of
affiliare "to adopt a son," from L.
ad- "to" +
filius "son" (see
filial). Fig. sense of "adoption by a society, of branches" first recorded 1799 (
affiliate in this sense is from 1761).