boy vs bat boy

boy and bat boy both are nouns.

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

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

boy: a youthful male person
bat boy: (baseball) a boy who takes care of bats and other baseball equipment

boy (noun)	bat boy (noun)
a youthful male person	(baseball) a boy who takes care of bats and other baseball equipment
(ethnic slur) offensive and disparaging term for Black man
a friendly informal reference to a grown man
a male human offspring