unnie vs adult female

unnie and adult female both are nouns.

Word	Noun	Adjective	Verb	Adverb
unnie	Yes	No	No	No
adult female	Yes	No	No	No

As nouns, adult female is a hypernym of unnie; that is, adult female is a word with a broader meaning than unnie:

unnie: a female who is older than the speaker, especially used by Korean female
adult female: an adult female person (as opposed to a man)

Other hypernyms of unnie include woman.

unnie (noun)	adult female (noun)
a female who is older than the speaker, especially used by Korean female	an adult female person (as opposed to a man)