Late 15th century; earliest use found in The Cely Papers: selections from the correspondence and memoranda of the Cely Family, Merchants of the Staple. From classical Latin socius companion, comrade, partner, colleague, accomplice, ally, use as noun of masculine of socius that keeps company with another, of or belonging to a partner or companion, perhaps from the same Indo-European base as Sanskrit sakhi, Avestan haxi companion, and probably more remotely also segge. The base may be further related to classical Latin sequī to follow.