''turn Turk'' - go bad; go against me.  The Turks were enemies of Europe, so the idea is of "going against."  For example, most of Hungary was under the control of the Turks by 1541.  When Shakespeare was born, Suleiman the Magnificant ruled the Ottoman Empire.  The Siege of Vienna was in 1529.  Vienna is mentioned in the play.  Hamlet is speaking of Fortune being his antagonist.
 
''turn Turk'' - go bad; go against me.  The Turks were enemies of Europe, so the idea is of "going against."  For example, most of Hungary was under the control of the Turks by 1541.  When Shakespeare was born, Suleiman the Magnificant ruled the Ottoman Empire.  The Siege of Vienna was in 1529.  Vienna is mentioned in the play.  Hamlet is speaking of Fortune being his antagonist.
  
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