Green’s Dictionary of Slang

(US) an idiot.

1965		T. Wolfe Kandy-Kolored Tangerine-Flake Streamline Baby (1966) x: Lower class creeps and nutballs with dermatitic skin and ratty hair.
1971		A. Prior Contract 110: He was a nutball but he may have been right.
1987		T. Wolfe Bonfire of the Vanities 364: Half a them the usual nutballs.