fatten up vs plump

fatten up is not a noun while plump is a noun.

Word	Noun	Adjective	Verb	Adverb
fatten up	No	No	Yes	No
plump	Yes	Yes	Yes	Yes

As verbs, fatten up and plump are synonyms defined as:

Other synonyms of fatten up include fat, fatten, fatten out, fill out, flesh out, plump out.

fatten up (noun)	plump (noun)
	the sound of a sudden heavy fall

fatten up (adjective)	plump (adjective)
	sufficiently fat so as to have a pleasing fullness of figure

fatten up (verb)	plump (verb)
make fat or plump	give support (to) or make a choice (of) one out of a group or number
	make fat or plump
	set (something or oneself) down with or as if with a noise
	drop sharply

fatten up (adverb)	plump (adverb)
	straight down especially heavily or abruptly