cowgirl vs adult female

cowgirl and adult female both are nouns.

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

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

Other hypernyms of cowgirl include cowhand, cowherd, cowpoke, cowpuncher, puncher, woman.

cowgirl (noun)	adult female (noun)
a woman cowboy	an adult female person (as opposed to a man)