set down vs plunk down

set down and plunk down both are verbs.

Word	Noun	Adjective	Verb	Adverb
set down	No	No	Yes	No
plunk down	No	No	Yes	No

As verbs, plunk down is a hyponym of set down; that is, plunk down is a word with a more specific, narrower meaning than set down:

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

set down (verb)	plunk down (verb)
put down in writing; of texts, musical compositions, etc.	set (something or oneself) down with or as if with a noise
remove (cargo, people, etc.) from and leave
put or settle into a position
cause to sit or seat or be in a settled position or place
go ashore
reach or come to rest