The Collaborative International Dictionary of English v.0.48:

Forcer \For"cer\, n.
   1. One who, or that which, forces or drives.
      [1913 Webster]

   2. (Mech.)
      (a) The solid piston of a force pump; the instrument by
          which water is forced in a pump.
      (b) A small hand pump for sinking pits, draining cellars,
          etc.
          [1913 Webster]