also partizan, 1550s, "one who takes part with another, zealous supporter," especially one whose judgment is clouded by prejudiced adherence to a party, from 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").
In military use, "member of a detachment of troops sent on a special mission," from 1690s. As these commonly were irregular troops, it took on the sense of "guerrilla fighter" in the Peninsular campaign of the Napoleonic wars and again in reference to resistance to Nazi occupation in the Balkans and Eastern Europe in World War II.
1708 in a military sense, "engaged on a special enterprise;" 1842 in politics, "of or pertaining to a party or faction;" from partisan (n.).