decline vs pooh-pooh

decline is a noun but pooh-pooh is not a noun.

Word	Noun	Adjective	Verb	Adverb
decline	Yes	No	Yes	No
pooh-pooh	No	No	Yes	No

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

Other hyponyms of decline include disdain, freeze off, reject, scorn, spurn, turn down, contract out, regret, repudiate, disobey.

decline (noun)	pooh-pooh (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)	pooh-pooh (verb)
grow smaller	reject with contempt
grow worse	express contempt about
show unwillingness towards
inflect for number, gender, case, etc.
go down
fall in value
not accept as true