Source : Webster's Revised Unabridged Dictionary (1913)

Plug \Plug\, n.

   {Breech plug} (Gun.), in breech-loading guns, the metal plug
      or cylinder which closes the aperture in the breech,
      through which the gun is loaded. Plug board \Plug board\
   (Elec.)
   A switchboard in which connections are made by means of
   plugs.