fun vs facetiousness

fun and facetiousness both are nouns.

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

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

fun (noun)	facetiousness (noun)
activities that are enjoyable or amusing	playful humor
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)