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.
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.