early 15c., "inclined to fighting," from Latin bellicosus "warlike, valorous, given to fighting," from bellicus "of war," from bellum "war" (Old Latin duellum, dvellum), which is of uncertain origin.
The best etymology for duellum so far has been proposed by Pinault 1987, who posits a dim. *duelno- to bonus. If *duelno- meant 'quite good, quite brave', its use in the context of war (bella acta, bella gesta) could be understood as a euphemism, ultimately yielding a meaning 'action of valour, war' for the noun bellum. [de Vaan]