last word vs nattiness

last word and nattiness both are nouns.

Word	Noun	Adjective	Verb	Adverb
last word	Yes	No	No	No
nattiness	Yes	No	No	No

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

Other hyponyms of last word include dapperness, jauntiness, rakishness.

last word (noun)	nattiness (noun)
elegance by virtue of being fashionable	stylishness as evidenced by a smart appearance
the final statement in a verbal argument
an authoritative statement