The Collaborative International Dictionary of English v.0.48:

Bilge \Bilge\, n. [A different orthography of bulge, of same
   origin as belly. Cf. Belly, Bulge.]
   1. The protuberant part of a cask, which is usually in the
      middle.
      [1913 Webster]

   2. (Naut.) That part of a ship's hull or bottom which is
      broadest and most nearly flat, and on which she would rest
      if aground.
      [1913 Webster]

   3. Bilge water.
      [1913 Webster]

   Bilge free (Naut.), stowed in such a way that the bilge is
      clear of everything; -- said of a cask.

   Bilge pump, a pump to draw the bilge water from the gold of
      a ship.

   Bilge water (Naut.), water which collects in the bilge or
      bottom of a ship or other vessel. It is often allowed to
      remain till it becomes very offensive.

   Bilge ways, the timbers which support the cradle of a ship
      upon the ways, and which slide upon the launching ways in
      launching the vessel.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bilge water
    n 1: water accumulated in the bilge of a ship [syn: bilge,
         bilge water]