feed vs fatten up

feed is a noun but fatten up is not a noun.

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

feed (noun)	fatten up (noun)
food for domestic livestock

feed (verb)	fatten up (verb)
provide with fertilizers or add nutrients to	make fat or plump
introduce continuously
take in food; used of animals only
give food to
serve as food for; be the food for
feed into; supply
provide as food
gratify
profit from in an exploitatory manner
move along, of liquids
support or promote