If your Uncle Marvin has a habit of telling corny jokes, they probably make you groan every single time. Corny describes something that’s either worn out and tiresome or overly sentimental — and sometimes both.
It can be hard to talk about something emotional without becoming corny. Try describing love at first sight without becoming trite or stickily sentimental and you’ll know first-hand how easily you can find yourself sounding corny. For years corny simply meant “full of corn,” or “tasting of malt.” Sometime in the 1930s it became popular slang meaning “appealing to country folk,” who were also described at the time as corn-fed, because of their supposed connection to farms.