fatten out vs plump

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

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

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

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

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

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

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