early 12c., "a sovereign prince," from Old French duc (12c.) and directly from Latin dux (genitive ducis) "leader, commander," in Late Latin "governor of a province," from ducere "to lead," from PIE root *deuk- "to lead." It is thus related to the second element in German Herzog "duke," Old English heretoga.
Applied in English to "hereditary nobleman of the highest rank" probably first mid-14c., ousting native earl. Also used to translate various European titles (such as Russian knyaz), usually of nobles ranking below a prince, but it was a sovereign title in some small states such as Burgundy, Normandy, and Lorraine.