grass widow vs adult female

grass widow and adult female both are nouns.

Word	Noun	Adjective	Verb	Adverb
grass widow	Yes	No	No	No
adult female	Yes	No	No	No

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

Other hypernyms of grass widow include woman.

grass widow (noun)	adult female (noun)
a divorced woman or a woman who is separated from her husband	an adult female person (as opposed to a man)