/ djuːk; NAmE duːk/ noun
1.
a nobleman of the highest rank :
the Duke of Edinburgh
2.
(in some parts of Europe, especially in the past) a male ruler of a small independent state
—see also archduke , duchess , duchy , grand duke
For more information see the Cultural Guide
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WORD ORIGIN
Old English (denoting the ruler of a duchy), from Old French duc , from Latin dux , duc- leader; related to ducere to lead.