Wordnet 3.0
NOUN (2)
1. 
 a military engineer who lays or detects and disarms mines; 
2. 
 a military engineer who does sapping (digging trenches or undermining fortifications); 
The Collaborative International Dictionary of English v.0.48:
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.
   [1913 Webster]
WordNet (r) 3.0 (2006):
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)