arcus

Arcus, arcus, m., fourth declension: nominative and genitive singular look identical, dative and ablative are arcu, and the plural is arcus again. Fourth-declension nouns are mostly masculine, with manus and domus the famous feminine exceptions; arcus is not one of them.

The sense travels outward from the weapon to anything bent: the arch of a building, the arc of a curve, and the rainbow, which Latin calls arcus caelestis or simply arcus. Note that the archer is sagittarius, from sagitta, "arrow", not from this word - Latin names him by what he shoots, not what he shoots it with. English arc, arch, archer and arcade all come down this line through French, and the arcus of a Roman aqueduct is the same object as the arch of one.