Green’s Dictionary of Slang

1. to make a bad joke; thus cornballing n.

1956		‘Ed Lacy’ Men from the Boys (1967) 108: ‘It was so hot last night I stuck me head in the ice-cube tray,’ I corn-balled.
1958		‘Ed Lacy’ Room to Swing 26: It’s too early for cornballing. What do you want?

2. to play music in a sentimental, unsophisticated way.

1956

‘James Updyke’ [W.R. Burnett] It’s Always Four O’Clock 110: ‘Okay, fellows. “Tiger Rag.” And let’s cornball it.’ Well, words fail! This time it was the end. We took it fast, hot and corny.