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c.1200, from Old English feolaga "fellow, partner," from Old Norse felagi, from fe "money" (see fee) + verbal base denoting "lay" (see lay (v.)). Sense is of "one who puts down money with another in a joint venture." Used familiarly since mid-15c. for "man, male person," but not etymologically masculine.
University senses (mid-15c.), corresponding to Latin socius) evolved from notion of "one of the corporation who constitute a college" and who are paid from its revenues. First record of fellow-traveler in sense of "one who sympathizes with the Communist movement but is not a party member," is from 1936, translating Russian poputchik. The literal sense is attested in English from 1610s.