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1570s, "full of corn, pertaining to corn, from corn (n.1) + -y (2). Chaucer used it of ale (late 14c.), perhaps to mean "malty." American English slang "old-fashioned, sentimental" is from 1932 (first attested in "Melody Maker"), perhaps originally "something appealing to country folk" (corn-fed in the same sense is attested from 1929). Related: Cornily; corniness.
Overly sentimental; banal; devoted to or expressing old-fashioned moral convictions; cornball
[1930+ Jazz musicians; the writer Mari Sandoz (1896–1966) suggested as possible origin the corn-seed catalogs sent to Midwestern farmers before and after 1900, which were larded with tired old jokes; the jokes were called corn jokes and corny]