keep vs keep down

keep is a noun but keep down is not a noun.

Word	Noun	Adjective	Verb	Adverb
keep	Yes	No	Yes	No
keep down	No	No	Yes	No

keep (noun)	keep down (noun)
the main tower within the walls of a medieval castle or fortress
a cell in a jail or prison
the financial means whereby one lives

keep (verb)	keep down (verb)
prevent (food) from rotting	manage not to throw up
prevent the action or expression of	place a limit on the number of
stick to correctly or closely	put down by force or intimidation
maintain by writing regular records
supply with necessities and support
hold and prevent from leaving
retain possession of
look after; be the keeper of; have charge of
retain rights to
maintain for use and service
have as a supply
store or keep customarily
stop (someone or something) from doing something or being in a certain state
allow to remain in a place or position or maintain a property or feature
conform one's action or practice to
behave as expected during of holidays or rites
cause to continue in a certain state, position, or activity
supply with room and board
maintain in safety from injury, harm, or danger
fail to spoil or rot
continue a certain state, condition, or activity
to rear