The Collaborative International Dictionary of English v.0.48:

Sap \Sap\, n. (Mil.)
   A narrow ditch or trench made from the foremost parallel
   toward the glacis or covert way of a besieged place by
   digging under cover of gabions, etc.
   [1913 Webster]

   Sap fagot (Mil.), a fascine about three feet long, used in
      sapping, to close the crevices between the gabions before
      the parapet is made.

   Sap roller (Mil.), a large gabion, six or seven feet long,
      filled with fascines, which the sapper sometimes rolls
      along before him for protection from the fire of an enemy.
      [1913 Webster]