Green’s Dictionary of Slang

nutball n.

[nut n.2 (1) + -ball sfx]

(US) an idiot.

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