plonk vs plump down

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

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

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

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

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

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

plonk (verb)	plump down (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