selectman vs adult male

selectman and adult male both are nouns.

Word	Noun	Adjective	Verb	Adverb
selectman	Yes	No	No	No
adult male	Yes	No	No	No

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

selectman: a male elected member of a board of officials who run New England towns
adult male: an adult person who is male (as opposed to a woman)

Other hypernyms of selectman include elected official, man.

selectman (noun)	adult male (noun)
a male elected member of a board of officials who run New England towns	an adult person who is male (as opposed to a woman)