proper noun/du/
Doubs (a left tributary of the Saône, running through the cantons of Neuchâtel and Jura, Switzerland and the departments of Doubs, Jura and Saône-et-Loire, France); Doubs (a department in Bourgogne-Franche-Comté, France)
From Gaulish Dubis, from the root of Proto-Celtic *dubus (“black”). Compare Latin Dūbis, Breton du (“black”).