The Collaborative International Dictionary of English v.0.48:

Nosebag \Nose"bag`\, n.
   A bag in which feed for a horse, ox, or the like, may be
   fastened under the nose by a string passing over the head.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nosebag
    n 1: a canvas bag that is used to feed an animal (such as a
         horse); covers the muzzle and fastens at the top of the
         head [syn: nosebag, feedbag]