plop

(noun) the noise of a rounded object dropping into a liquid without a splash.

(verb) drop something with a plopping sound.

(verb) drop with the sound of something falling into water.

(verb) set (something or oneself) down with or as if with a noise. Synonyms: plank, flump, plonk, plunk, plump down, plunk down, plump. Examples: "He planked the money on the table." "He planked himself into the sofa."

(adv) with a short hollow thud. Synonyms: plunk. Example: "Plop came the ball down to the corner of the green."