The Collaborative International Dictionary of English v.0.48:

Plop \Plop\ (pl[o^]p), v. i. [imp. & p. p. Plopped (pl[o^]pt);
   p. pr. & vb. n. Plopping.] [Imitative.]
   1. To fall, drop, or move in any way, with a sudden splash or
      slap, as on the surface of water.
      [Webster 1913 Suppl.]

            The body plopped up, turning on its side. --Kipling.
      [Webster 1913 Suppl.]