partisan (adj.)

1708 for warfare, 1842 for politics, from partisan (n.).

partisan (n.)

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) "a part, piece, a share, a division; a party or faction; a part of the body; a fraction; a function, office" (from PIE root *pere-(2) "to grant, allot"). Sense of "guerrilla fighter" is first recorded 1690s.