The Collaborative International Dictionary of English v.0.48:

Slop \Slop\, n. [OE. sloppe a pool; akin to As. sloppe, slyppe,
   the sloppy droppings of a cow; cf. AS. sl?pan to slip, and E.
   slip, v.i. Cf. Cowslip.]
   1. Water or other liquid carelessly spilled or thrown aboyt,
      as upon a table or a floor; a puddle; a soiled spot.
      [1913 Webster]

   2. Mean and weak drink or liquid food; -- usually in the
      plural.
      [1913 Webster]

   3. pl. Dirty water; water in which anything has been washed
      or rinsed; water from wash-bowls, etc.
      [1913 Webster]

   Slop basin, or Slop bowl, a basin or bowl for holding
      slops, especially for receiving the rinsings of tea or
      coffee cups at the table.

   Slop molding (Brickmaking), a process of manufacture in
      which the brick is carried to the drying ground in a wet
      mold instead of on a pallet.
      [1913 Webster]

WordNet (r) 3.0 (2006):

slop bowl
    n 1: a bowl into which the dregs of teacups and coffee cups are
         emptied at the table [syn: slop basin, slop bowl]