still vs calm down

still is a noun but calm down is not a noun.

Word	Noun	Adjective	Verb	Adverb
still	Yes	Yes	Yes	Yes
calm down	No	No	Yes	No

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

Other hyponyms of still include shush, lull, shout down, gag, muzzle.

As verbs, still and calm down are synonyms defined as:

Other synonyms of still include calm, lull, quiet, quieten, tranquilize, tranquillise, tranquillize.

still (noun)	calm down (noun)
an apparatus used for the distillation of liquids; consists of a vessel in which a substance is vaporized by heat and a condenser where the vapor is condensed
a static photograph (especially one taken from a movie and used for advertising purposes)
a plant and works where alcoholic drinks are made by distillation
(poetic) tranquil silence

still (adjective)	calm down (adjective)
not sparkling
used of pictures; of a single or static photograph not presented so as to create the illusion of motion; or representing objects not capable of motion
(of a body of water) free from disturbance by heavy waves
marked by absence of sound
not in physical motion
free from noticeable current

still (verb)	calm down (verb)
cause to be quiet or not talk	become quiet or less intensive
make motionless	make calm or still
make calm or still	become quiet or calm, especially after a state of agitation
lessen the intensity of or calm

still (adverb)	calm down (adverb)
to a greater degree or extent; used with comparisons
with reference to action or condition; without change, interruption, or cessation
without moving or making a sound
despite anything to the contrary (usually preceding a concession)