Source : Webster's Revised Unabridged Dictionary (1913)

Boyau \Boy"au\, n.; pl. {Boyaux} or {Boyaus}. [F. boyau gut, a
   long and narrow place, and (of trenches) a branch. See
   {Bowel}.] (Fort.)
   A winding or zigzag trench forming a path or communication
   from one siegework to another, to a magazine, etc.