In Roman law. A leader or military commander. The commander of an army.Dig. 3, 2, 2, pr.In feudal and old European law. Duke; a title of honor, or order of nobility. 1 Bl.Comm. 397; Crabb, Eng. Law, 236.In later law. A military governor of a province. See Cod. 1, 27, 2. A military officerhaving charge of the borders or frontiers of the empire, called “dux Umitis.”‘ Cod. 1, 49,1, pr. At this period, the word began to be used as a title of honor or dignity.