The Collaborative International Dictionary of English v.0.48:

Stomacher \Stom"ach*er\, n.
   1. One who stomachs.
      [1913 Webster]

   2. (? or ?) An ornamental covering for the breast, worn
      originally both by men and women. Those worn by women were
      often richly decorated.
      [1913 Webster]

            A stately lady in a diamond stomacher. --Johnson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

stomacher
    n 1: garment consisting of a V-shaped panel of stiff material
         worn over the chest and stomach in the 16th century

Easton's 1897 Bible Dictionary:

Stomacher
   (Isa. 3:24), an article of female attire, probably some sort of
   girdle around the breast.