The Collaborative International Dictionary of English v.0.48:

Waistcoat \Waist"coat\, n.
      (a) A short, sleeveless coat or garment for men, worn
          under the coat, extending no lower than the hips, and
          covering the waist; a vest.
      (b) A garment occasionally worn by women as a part of
          fashionable costume.
          [1913 Webster]

   Note: The waistcoat was a part of female attire as well as
         male . . . It was only when the waistcoat was worn
         without a gown or upper dress that it was considered
         the mark of a mad or profligate woman. --Nares.
         [1913 Webster]

   Syn: See Vest.
        [1913 Webster]

WordNet (r) 3.0 (2006):

waistcoat
    n 1: a man's sleeveless garment worn underneath a coat [syn:
         vest, waistcoat]