The Collaborative International Dictionary of English v.0.48:

Pendragon \Pen"drag*on\, n.
   A chief leader or a king; a head; a dictator; -- a title
   assumed by the ancient British chiefs when called to lead
   other chiefs.
   [1913 Webster]

         The dread Pendragon, Britain's king of kings.
                                                  --Tennyson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pendragon
    n 1: the supreme war chief of the ancient Britons