barbarus

Barbarus, barbara, barbarum declines like bonus, and in the masculine plural it works as a straight noun: barbari, "the barbarians". Latin borrowed it from Greek barbaros, which was mockery of how foreign speech sounded to a Greek ear, all bar-bar and no sense.

That origin creates the useful trap. To a Greek, every non-Greek was a barbaros, Romans very much included. The Romans took the word, quietly exempted themselves, and used it for everyone outside the Graeco-Roman world. For a classical writer it therefore means "foreign" first, with no necessary insult - Plautus can call Latin itself barbara lingua when translating a Greek play. The hardening into "savage, brutal" is later, and by the time medieval chroniclers are writing about invasions it has set solid.