hollow out vs hollow

hollow out is not a noun while hollow is a noun.

Word	Noun	Adjective	Verb	Adverb
hollow out	No	No	Yes	No
hollow	Yes	Yes	Yes	No

As verbs, hollow out and hollow are synonyms defined as:

Other synonyms of hollow out include core out.

hollow out (noun)	hollow (noun)
	a depression hollowed out of solid matter
	a small valley between mountains
	a cavity or space in something

hollow out (adjective)	hollow (adjective)
	not solid; having a space or gap or cavity
	lacking in substance or character
	devoid of significance or force
	as if echoing in a hollow space

hollow out (verb)	hollow (verb)
remove the interior of	remove the interior of
	remove the inner part or the core of