The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]