duke (n.) Look up duke at Dictionary.com
early 12c., "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." Applied in English to "nobleman of the highest rank" probably first mid-14c., ousting native earl. Also used to translate various European titles (such as Russian knyaz).