Green’s Dictionary of Slang

a naïve, unsophisticated person.

1952		R.C. Ruark 17 Mar. [synd. col.] Eisenhower on no account can be called a cornball [W&F].
1960		Mad mag. Sept. 44: Man, once I was a real ‘square.’ I mean once I was a real cornball.
1966		T. Keyes All Night Stand 85: Real Kansas cornballs and as needy for a fist in the face as anyone.
1981		J. Ellroy Brown’s Requiem 135: I always advanced the opinion, derived from Walter, that Chopin was a cornball and a sentimentalist.
1987		W.T. Vollmann You Bright and Risen Angels (1988) 78: Mr. White would call us cornballs and darned old sourpusses.
1997		C. Hiaasen Lucky You 146: A cornball, this guy.
2001		Source Aug. 68: I think the dude from Sparks who played Juwanna is a cornball, but he was good for this role.
2011		L.A. Downtown News 5 Dec. 28/3: Only a real cornball would fall for the black magic scam.