drop down vs sag

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

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

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

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

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

drop down (verb)	sag (verb)
fall or descend to a lower place or level	droop, sink, or settle from or as if from pressure or loss of tautness
	cause to sag