Source : Webster's Revised Unabridged Dictionary (1913)

Plop \Plop\, v. i. [imp. & p. p. {Plopped}; p. pr. & vb. n.
   {Plopping}.] [Imitative.]
   To fall, drop, or move in any way, with a sudden splash or
   slap, as on the surface of water.

         The body plopped up, turning on its side. --Kipling.

Plop \Plop\, n.
   Act of plopping; the sound made in plopping.

Source : WordNet®

plop
     n : the noise of a rounded object dropping into a liquid without
         a splash
     adv : with a short hollow thud; "plop came the ball down to the
           corner of the green" [syn: {plunk}]
     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}, {plunk}, {plump
        down}, {plunk down}, {plump}]
     [also: {plopping}, {plopped}]