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 *deuk- "to lead" (cf. Old English togian "to pull, drag," Old High German ziohan "to pull," Old English togian "to draw, drag," Middle Welsh dygaf "I draw").
Applied in English to "nobleman of the highest rank" probably first mid-14c., ousting native earl. Also used to translate various European titles (e.g. Russian knyaz).