Nouns

(n)plopthe noise of a rounded object dropping into a liquid without a splash

Verbs

(v)plopdrop something with a plopping sound
(v)plopdrop with the sound of something falling into water
(v)plank, flump, plonk, plop, plunk, plump down, plunk down, plumpset (something or oneself) down with or as if with a noise“He planked the money on the table”, “He planked himself into the sofa”

Adverbs

(r)plop, plunkwith a short hollow thud“plop came the ball down to the corner of the green”