From 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]