The Collaborative International Dictionary of English v.0.48:

Tampion \Tam"pi*on\, n. [F. tampon, tapon, tape, of Dutch or
   German origin. See Tap a pipe or plug, and cf. Tamp,
   Tampop, Tompion.] [Written also tampeon, and
   tompion.]
   1. A wooden stopper, or plug, as for a cannon or other piece
      of ordnance, when not in use.
      [1913 Webster]

   2. (Mus.) A plug for upper end of an organ pipe.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tampion
    n 1: plug for the muzzle of a gun to keep out dust and moisture
         [syn: tampion, tompion]