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.]

The Collaborative International Dictionary of English v.0.48:

plop \plop\ v. t.
   1. to drop (something) so that it makes a plopping sound;
      especially, to drop with the sound of something falling
      into water. [WordNet sense 1, 2]
      [WordNet 1.5]

   2. To set down quickly, so as to make a sound; -- used often
      with the reflexive; as, He plopped himself into the sofa.
      [WordNet sense 3]

   Syn: plank, flump, plonk, plunk, plump down, plunk down,
        plump.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Plop \Plop\, n.
   Act of plopping; the sound made in plopping.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

plop
    adv 1: with a short hollow thud; "plop came the ball down to the
           corner of the green" [syn: plop, plunk]
    n 1: the noise of a rounded object dropping into a liquid
         without a splash
    v 1: drop something with a plopping sound
    2: drop with the sound of something falling into water
    3: set (something or oneself) down with or as if with a noise;
       "He planked the money on the table"; "He planked himself into
       the sofa" [syn: plank, flump, plonk, plop, plunk,
       plump down, plunk down, plump]