Corny

In (American?) English slang, that which is cliched, overly sentimental, insincere, kitschy, lame, cheesy, and so on. Has given way in popularity to cheesy. examples: a corny joke, a corny tourist trap music show, a corny Hallmark card.

Q: Why did the cow cross the road?
A: To get to the udder side!*
(Ok, you can stop cringing....)

Everyone knows a corny joke when they hear one. However, while the meaning of “corny” is clear, its origin remains uncertain.
Two conflicting theories explain its entrance into American vernacular.

The first theory: “Corny” was born during the 1890s via a seed catalogue sent to farmers. The catalogue included seed corn and became notorious for the short jokes, riddles, and cartoons scattered throughout the publication. These trite, unsophisticated, and often out-dated attempts at comedy were termed “corn jokes” and soon all similarly banal quips were dubbed “corny”.
The second theory: When traveling vaudeville acts passed through the Corn Belt, situated in the midwestern United States, they encountered locals who appreciated straightforward, conventional comedy. Comedians of the era coined the phrase “corn-fed humor” (which evolved into the adjective in question by the 1930s) since corn was an important part of their audience’s diet.

“Corny” appeared in a 1932 issue of Melody Maker, an American music magazine. "The 'bounce' of the brass section... has degenerated into a definitely 'corny' and staccato style of playing." The quotation marks surrounding “corny” denote its newness.

Sources:
Eat Your Words by Charlotte Foltz Jones
http://www.aboutri.com/Hospitality/Restaurants/Articles/SpecialInterest/Maize/ArticleMaizePart1TheGiftOfMaize.htm
http://www.takeourword.com/TOW185/page2.html

Cor"ny (k?r"n?), a. [L. cornu horn.]

Strong, stiff, or hard, like a horn; resembling horn.

Up stood the cornu reed. Milton.

© Webster 1913.

Corn"y, a.

1.

Producing corn or grain; furnished with grains of corn.

Prior.

2.

Containing corn; tasting well of malt.

A draught of moist and corny ale. Chaucer.

3.

Tipsy.

Forby.

© Webster 1913.