1. [n]  a British peer of the highest rank
2. [n]  a nobleman (in various countries) of high rank

DUKE is a 4 letter word that starts with D.

1. \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}.
   
   

2. \Duke\, v. i.
   To play the duke. [Poetic]
   
         Lord Angelo dukes it well in his absence. -- Shak.
   
   

derived from the Latin dux, meaning "a leader;" Arabic, "a sheik." This word is used to denote the phylarch or chief of a tribe (Gen. 36:15-43; Ex. 15:15; 1 Chr. 1:51-54).