The authors agree that the base of the surname is the term guerra ("war"), documented from the 12th century in the Latinized forms Guerra, Guerraccius and Guerrerius. A. Bongioanni considers it mainly a shortened form of medieval compound names alluding to birth or expressing an omen: Buonaguerra, Nascinguerra, Finiguerra and Vinciguerra, all of which became surnames. E. De Felice adds that the surname may also derive directly from the noun, with various motivations.

E. De Stefani specifies that Nascinguerra meant "born in war" and was given to a child born in a time of conflict, following the medieval practice of naming the child after the period in which he had been born. He further notes that from at least the 12th century Guerra was used as an independent nickname, attested in Friuli-Venezia Giulia from the 14th century, until it became a surname.

E. Rossoni proposes that the surname may derive from the Germanic name Vhuerius, which evolved into Guerius and then into the medieval Guerra, whose ultimate root would be the Frankish werra ("strife"). This Germanic etymology is also developed by V. Blunda, who links it specifically to the Lombard personal name Werra and notes that the term displaced, in most of the Romance lexicon, the Latin bellum, preserved only in learned derivatives such as "bellicose" or "belligerent". Blunda adds that the variants Guarrasi and Guerrasio, proper to Sicily, reflect dialect phonetic adaptations of the original personal name.

The surname is found throughout continental Italy, with a greater presence in the north according to Rossoni. The variant Guerrazzi predominates in Tuscany, while Guerrera is characteristic of Sicily.