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also partizan, 1550s, "one who takes part with another, zealous supporter," from Middle French partisan (15c.), from dialectal upper Italian partezan (Tuscan partigiano) "member of a faction, partner," from parte "part, party," from Latin partem (nominative pars), see part (n.). Sense of "guerilla fighter" is first recorded 1690s.
1708 for warfare, 1842 for politics, from partisan (n.).