"Nut" as "head" makes sense. And it's still current; if you strike someone in the face with your forehead, you have nutted them. So I buy "nutcase" as "head-ca

"Nut" as "head" makes sense. And it's still current; if you strike someone in the face with your forehead, you have nutted them. So I buy "nutcase" as "head-case", therefore "mad".