Why is the ninth month called September?
1590s (from late 13c. in Latin form), from Medieval Latin duellum "combat between two persons," by association with Latin duo "two," but originally from Latin duellum "war," an Old Latin form of bellum (see bellicose). Retained in poetic and archaic language and apparently given a special meaning in Medieval or Late Latin of "one-on-one combat" on fancied connection with duo "two."
1640s, see duel (n.). Related: Dueled; dueling; duelling.