drop down vs droop

drop down is not a noun while droop is a noun.

Word	Noun	Adjective	Verb	Adverb
drop down	No	No	Yes	No
droop	Yes	No	Yes	No

As verbs, droop is a hyponym of drop down; that is, droop is a word with a more specific, narrower meaning than drop down:

drop down: fall or descend to a lower place or level
droop: droop, sink, or settle from or as if from pressure or loss of tautness

Other hyponyms of drop down include drop open, fall open, flag, sag, swag, sag, sag down.

drop down (noun)	droop (noun)
	a shape that sags

drop down (verb)	droop (verb)
fall or descend to a lower place or level	become limp
	droop, sink, or settle from or as if from pressure or loss of tautness
	hang loosely or laxly