Source : Webster's Revised Unabridged Dictionary (1913)

Duke \Duke\n. [F. duc, fr. L. dux, ducis, leader, commander, fr.
   ducere to lead; akin to AS. te['o]n to draw; cf. AS. heretoga
   (here army) an army leader, general, G. herzog duke. See
   {Tue}, and cf. {Doge}, {Duchess}, {Ducat}, {Duct}, {Adduce},
   {Deduct}.]
   1. A leader; a chief; a prince. [Obs.]

            Hannibal, duke of Carthage.           --Sir T.
                                                  Elyot.

            All were dukes once, who were ``duces'' -- captains
            or leaders of their people.           --Trench.

   2. In England, one of the highest order of nobility after
      princes and princesses of the royal blood and the four
      archbishops of England and Ireland.

   3. In some European countries, a sovereign prince, without
      the title of king.

   {Duke's coronet}. See {Illust}. of {Coronet}.

   {To dine with Duke Humphrey}, to go without dinner. See under
      {Dine}.

Duke \Duke\, v. i.
   To play the duke. [Poetic]

         Lord Angelo dukes it well in his absence. -- Shak.

Source : WordNet®

duke
     n 1: a British peer of the highest rank
     2: a nobleman (in various countries) of high rank