The Collaborative International Dictionary of English v.0.48:

Bossage \Boss"age\, n. [F. bossage, fr. bosse. See Boss a
   stud.]
   1. (Arch.) A stone in a building, left rough and projecting,
      to be afterward carved into shape. --Gwilt.
      [1913 Webster]

   2. (Arch.) Rustic work, consisting of stones which seem to
      advance beyond the level of the building, by reason of
      indentures or channels left in the joinings. --Gwilt.
      [1913 Webster]