Source : Webster's Revised Unabridged Dictionary (1913)

Breastsummer \Breast"sum`mer\, n. (Arch.)
   A summer or girder extending across a building flush with,
   and supporting, the upper part of a front or external wall; a
   long lintel; a girder; -- used principally above shop
   windows. [Written also {brestsummer} and {bressummer}.]