nattiness vs last word

nattiness and last word both are nouns.

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

As nouns, last word is a hypernym of nattiness; that is, last word is a word with a broader meaning than nattiness:

Other hypernyms of nattiness include chic, chichi, chicness, modishness, smartness, stylishness, swank.

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