The Collaborative International Dictionary of English v.0.48:

Slop \Slop\, v. t. [imp. & p. p. Slopped; p. pr. & vb. n.
   Slopping.]
   1. To cause to overflow, as a liquid, by the motion of the
      vessel containing it; to spill.
      [1913 Webster]

   2. To spill liquid upon; to soil with a liquid spilled.
      [1913 Webster]