undress vs strip down

undress is a noun but strip down is not a noun.

Word	Noun	Adjective	Verb	Adverb
undress	Yes	No	Yes	No
strip down	No	No	Yes	No

As verbs, undress and strip down are synonyms defined as:

Other synonyms of undress include discase, disrobe, peel, strip, uncase, unclothe.

undress (noun)	strip down (noun)
partial or complete nakedness

undress (verb)	strip down (verb)
get undressed	get undressed
remove (someone's or one's own) clothes