adult vs brute

adult and brute both are nouns.

Word	Noun	Adjective	Verb	Adverb
adult	Yes	Yes	No	No
brute	Yes	Yes	No	No

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

Other hypernyms of adult include animal, animate being, beast, creature, fauna.

adult (noun)	brute (noun)
any mature animal	a living organism characterized by voluntary movement
a fully developed person from maturity onward	a cruelly rapacious person

adult (adjective)	brute (adjective)
(of animals) fully developed	resembling a beast; showing lack of human sensibility
designed to arouse lust