The Collaborative International Dictionary of English v.0.48:

Main-gauche \Main`-gauche"\ (m[a^]N`g[=o]sh"), n. [F., the left
   hand.] (Ancient Armor)
   The dagger held in the left hand, while the rapier is held in
   the right; -- used to parry thrusts of the adversary's
   rapier.
   [1913 Webster]