brain vs calf's brain

brain and calf's brain both are nouns.

Word	Noun	Adjective	Verb	Adverb
brain	Yes	No	Yes	No
calf's brain	Yes	No	No	No

As nouns, calf's brain is a hyponym of brain; that is, calf's brain is a word with a more specific, narrower meaning than brain:

brain (noun)	calf's brain (noun)
that part of the central nervous system that includes all the higher nervous centers; enclosed within the skull; continuous with the spinal cord	the brain of a calf eaten as meat
mental ability
that which is responsible for one's thoughts, feelings, and conscious brain functions; the seat of the faculty of reason
the brain of certain animals used as meat
someone who has exceptional intellectual ability and originality

brain (verb)	calf's brain (verb)
hit on the head
kill by smashing someone's skull