Mostly used to describe jokes or comedy that is either not very funny or outdated or just a bitt silly not really a joke.

Or used to say something is not very original.

it means the comedy joke or whatever was said is, dull, ordinary, obvious.

a very corny joke is

"why did the chicken cross the road?" you reply I don't know why did the chicken cross the road?.

the joke teller replies back "to get to the other side"

Kinda complicated. But it is about comedy. When someone says a joke that is bad. Not bad as in mean or evil. Bad in a way where it sucked, it was a lame joke. It has nothing to do with the food corn. It is about bad jokes.