The Collaborative International Dictionary of English v.0.48:

Bridoon \Bri*doon"\, n. [F. bridon, from bride; of German
   origin. See Bridle, n.] (Mil.)
   The snaffle and rein of a military bridle, which acts
   independently of the bit, at the pleasure of the rider. It is
   used in connection with a curb bit, which has its own rein.
   --Campbell.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bridoon
    n 1: a bit resembling a snaffle bit; used with a separate curb