Green’s Dictionary of Slang

(US) a sodomite of men or women.

1990		(con. early 1950s) J. Ellroy L.A. Confidential 178: Film cans, smutty titles: [...] Cornholers Meet the Ramrod Boys.
1998		K. Anderson Night Dogs 95: ‘Back in New Jersey, where I grew up, you know what we called ’em [i.e. homosexuals], sergeant?’ ‘No sir.’ ‘We called ’em ‘cornholers’.