partisan

English partisan comes from French partisan, from Italian partigiano, meaning 'defender of a party,' itself built on parte, 'part.' It entered English in the late 1400s as a word for a party adherent.

By the 16th century the word had grown adjective senses describing party loyalty, and around 1690 it picked up a new meaning: a guerrilla fighter, a sense shaped by Serbo-Croatian partizan and Russian партизан, which trace back to the same Italian root. The military adjective sense followed in the early 1700s.

There's also an unrelated-looking twin: partisan meaning a spear-like weapon, from French partizaine and Italian partigiana — linked to the same Italian source, apparently because such a weapon was seen as typical gear for these fighters.