boy vs farm boy

boy and farm boy both are nouns.

Word	Noun	Adjective	Verb	Adverb
boy	Yes	No	No	No
farm boy	Yes	No	No	No

As nouns, farm boy is a hyponym of boy; that is, farm boy is a word with a more specific, narrower meaning than boy:

boy (noun)	farm boy (noun)
a youthful male person	a boy who has grown up on a farm
(ethnic slur) offensive and disparaging term for Black man
a friendly informal reference to a grown man
a male human offspring