frivolousness vs fun

frivolousness and fun both are nouns.

Word	Noun	Adjective	Verb	Adverb
frivolousness	Yes	No	No	No
fun	Yes	No	No	No

As nouns, fun is a hyponym of frivolousness; that is, fun is a word with a more specific, narrower meaning than frivolousness:

Other hyponyms of frivolousness include giddiness, silliness, levity, playfulness.

frivolousness (noun)	fun (noun)
the trait of being frivolous; not serious or sensible	activities that are enjoyable or amusing
	violent and excited activity
	a disposition to find (or make) causes for amusement
	verbal wit or mockery (often at another's expense but not to be taken seriously)