The Collaborative International Dictionary of English v.0.48:

diehard \die"hard`\ (d[imac]"h[aum]rd`), n.
   1. one who stubbornly adheres to traditional and outdated
      views.

   Syn: traditionalist.
        [WordNet 1.5]

   2. one who vigorously defends an apparently hopeless
      position, a lost cause, etc.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

diehard \die"hard`\ (d[imac]"h[aum]rd`), a.
   stubbornly and vigorously resisting in the face of seemingly
   hopeless odds; as, diehard opposition.

   Syn: never-say-die. [PJC]

WordNet (r) 3.0 (2006):

diehard
    n 1: one who adheres to traditional views [syn:
         traditionalist, diehard]