decline vs sicken

decline is a noun but sicken is not a noun.

Word	Noun	Adjective	Verb	Adverb
decline	Yes	No	Yes	No
sicken	No	No	Yes	No

As verbs, sicken is a hyponym of decline; that is, sicken is a word with a more specific, narrower meaning than decline:

decline (noun)	sicken (noun)
a downward slope or bend
a gradual decrease; as of stored charge or current
change toward something smaller or lower
a condition inferior to an earlier condition; a gradual falling off from a better state; decline

decline (verb)	sicken (verb)
grow smaller	get sick
grow worse	make sick or ill
show unwillingness towards	cause aversion in; offend the moral sense of
inflect for number, gender, case, etc.	upset and make nauseated
go down
fall in value
not accept as true