The Collaborative International Dictionary of English v.0.48:

Chapeau \Cha`peau"\, n.; pl. Chapeux. [F., fr. OF. chapel hat.
   See Chaplet.]
   1. A hat or covering for the head.
      [1913 Webster]

   2. (Her.) A cap of maintenance. See Maintenance.
      [1913 Webster]

   Chapeau bras[F. chapeau hat + bras arm], a hat so made that
      it can be compressed and carried under the arm without
      injury. Such hats were particularly worn on dress
      occasions by gentlemen in the 18th century. A chapeau bras
      is now worn in the United States army by general and staff
      officers.
      [1913 Webster]