1751, "adoption," from French affiliation, from Medieval Latin affiliationem (nominative affiliatio), noun of action from past-participle stem of Latin affiliare "to adopt as a son," from ad "to" (see ad-) + filius "son" (see filial). Figurative sense of "adoption by a society, of branches" first recorded 1799 (the verb affiliate in a related sense is from 1761). Meaning "friendship, relationship, association" is from 1852.
