duellum.

Etymology tree Proto-Indo-European *dewh₂-der.? Proto-Italic *dwenos Archaic Latin duenos Old Latin *duenelos? Latin duellum De Vaan 2008 favors Pinault 1987's proposed derivation from *duenelos, a diminutive of duenos, itself an earlier form of bonus (“good, brave”). Perhaps the sense of "brave" allowed for the term to become a euphemism for wartime deeds, facilitating an eventual generalization to the overall concept of war. Sihler explains the retention of -e- here as an effect of the following l exilis, arguing that *dwe- normally became *dwo- > bo- (compare duenos > bonus). Pokorny instead connected the word to terms such as Ancient Greek δαίω (daíō), which is now often derived from *deh₂w- (“to burn”). By a regular sound change (seen already in Plautus), dw- evolved in Latin to b-: this turned duellum into bellum, the usual form used for the sense "war". However, the archaic form duellum survived in poetry. In Medieval Latin, the sense shifted to a combat between, specifically, two contenders, under the influence of the (non-cognate) word duo (“two”).