The Collaborative International Dictionary of English v.0.48:

Drumhead \Drum"head`\, n.
   1. The parchment or skin stretched over one end of a drum.
      [1913 Webster]

   2. The top of a capstan which is pierced with sockets for
      levers used in turning it. See Illust. of Capstan.
      [1913 Webster]

   Drumhead court-martial (Mil.), a summary court-martial
      called to try offenses on the battlefield or the line of
      march, when, sometimes, a drumhead has to do service as a
      writing table.
      [1913 Webster]

WordNet (r) 3.0 (2006):

drumhead court-martial
    n 1: a military court convened to hear urgent charges of
         offenses committed in action