plump down vs plonk

plump down is not a noun while plonk is a noun.

Word	Noun	Adjective	Verb	Adverb
plump down	No	No	Yes	No
plonk	Yes	No	Yes	No

As verbs, plump down and plonk are synonyms defined as:

plump down and plonk: set (something or oneself) down with or as if with a noise

Other synonyms of plump down include flump, plank, plop, plump, plunk, plunk down.

plump down (noun)	plonk (noun)
	the noise of something dropping (as into liquid)
	a cheap wine of inferior quality

plump down (verb)	plonk (verb)
set (something or oneself) down with or as if with a noise	set (something or oneself) down with or as if with a noise
drop heavily