11th cent. Disputed etymology. Certainly from a Germanic source. Following Corominas (1991) and Roberts (2014), perhaps Proto-West Germanic *werra "confusion (of battle)," or a similar meaning.
From there, the traditional etymology is from Osthoff (1910) who posits an origin in Proto-Germanic *wersa- "confusion," "disruption," "greater evil." From Proto-Indo-European *u̯ers- "to confuse."
Also the origin of the surnames Guerra and Guerro.