1761, "bring into close association," from Latin affiliatus, past participle of affiliare "to adopt a son," from ad "to" (see ad-) + filius "son" (see filial). Outside legal use, always figurative. Related: Affiliated; affiliating.
"taken into close association," 1858, from affiliate (v.).
1846, from affiliate (v.) via the adjective. Compare associate (n.). Affiliated society in reference to a local society connected with another or associated with a central organization is from 1795.