Source : Webster's Revised Unabridged Dictionary (1913)

Sapper \Sap"per\, n. [Cf. F. sapeur.]
   One who saps; specifically (Mil.), one who is employed in
   working at saps, building and repairing fortifications, and
   the like.

Source : WordNet®

sapper
     n 1: a military engineer who lays or detects and disarms mines
     2: a military engineer who does sapping (digging trenches or
        undermining fortifications)