Source : Webster's Revised Unabridged Dictionary (1913)

Quadruplex \Quad"ru*plex\, a. [L., from quattuor four + plicare
   to fold.]
   Fourfold; folded or doubled twice.

   {Quadruplex system} (Electric Telegraph), a system by which
      four messages, two in each direction, may be sent
      simultaneously over the wire.