partisan

Etymology 1

From French partisan, from Italian partigiano (“defender of a party”), from parte (“part”). Doublet of partigiano. Attested in English from the late 15th century in the noun sense of "party adherent", and in related adjective senses from the 16th century. The "guerilla fighter" sense influenced by Serbo-Croatian partizan, Russian партиза́н (partizán), from the same source. The sense of "guerilla fighter" is from c. 1690. The adjective in the military sense dates from the early 18th century.

noun

1. An adherent to a party or faction.
   Strong partisans of neither party, Indiana farmers failed to act as a block […] 1992, Thomas R. Pegram, Partisans and Progressives: Private Interest and Public Policy in Illinois
2. A fervent, sometimes militant, supporter or proponent of a party, cause, faction, person, or idea.
3. A member of a band of detached light, irregular troops acting behind occupying enemy lines in the ways of harassment or sabotage; a guerrilla fighter.
4. (now rare) The commander of a body of detached light troops engaged in making forays and harassing an enemy.

adj

1. Serving as commander or member of a body of detached light troops.
   partisan officer, partisan corps
2. Adherent to a party or faction; especially, having the character of blind, passionate, or unreasonable adherence to a party.
   They were blinded by partisan zeal.
   Having put partisan interests to one side, Greenwood set about making teamwork a watchword for her chairmanship, while seeking the broadest possible participation in subsequent inquiries. June 3 2020, Lilian Greenwood talks to Paul Stephen, “Rail's 'underlying challenges' remain”, in Rail, page 33
3. Devoted to or biased in support of a party, group, or cause.
   partisan politics
   England will regard it as a measure of justice for Frank Lampard's disallowed goal against Germany in Bloemfontein at the 2010 World Cup - but it was also an illustration of how they rode their luck for long periods in front of a predictably partisan home crowd. June 19, 2012, Phil McNulty, “England 1-0 Ukraine”, in BBC Sport
   He finds, not “demigods” but “a combative group of exhausted, drunken, broken, petty, partisan, scheming, squabbling, bloviating, sensory-deprived, underoxygenated, fed-up, talked-out, overheated delegates so distraught they threatened violence, secession. 2013, The Ultimate Daily Show and Philosophy

Etymology 2

From French partizaine, from Middle French partizaine, partisanne etc., from Italian partigiana, related to Etymology 1 above (apparently because it was seen as a typical weapon of such forces).

noun

1. (historical) A long-handled spear with a triangular, double-edged blade having lateral projections, in some forms also used in boar hunting.
   Salisbury and his attendants were also now drawing near, with bills and partisans brandished, and bows already bended. 1825, Sir Walter Scott, The Talisman
2. (obsolete) A soldier armed with such a weapon.