The word corny comes from mail order seed catalogues that existed in the early 20th century. The author of these catalogues wanted to increase the profits of his book and interest people in reading it too. The publishers helped the author by mixing in cartoon, jokes and riddles throughout the crop and garden book. These intriguing inserts were known to be very bad, very very bad. Since that every bad jokes were called corny catalogue jokes. But then the jokes simply became corny jokes and corny became associated with anything an failed attempt to something amusing.
Did you know corny is actually known as Maize, which resembles a lot to the french word Maïs which looks a lot like the Latin word.