Inherited from Old Latin duonus, from earlier duenos, from Proto-Italic *dwenos, further etymology uncertain. Possibly cognate with beō (“to bless, gladden”). Possible roots include:
* Proto-Indo-European *dewh₂- (“to join, fit together”); compare Gothic 𐍄𐌰𐌿𐌾𐌰𐌽 (taujan, “to do, make”) and Ancient Greek δύναμαι (dúnamai, “to be able”).
* Proto-Indo-European *deh₃-u, from *deh₃- (“to give”).
* Proto-Indo-European *dew- (“to show favor, revere”)
* Proto-Indo-European *dwey- (“to fear”), source of Ancient Greek δέος (déos), δεινός (deinós), δειλός (deilós).
Compare the change from duellum to bellum, and from duis to bis.