Corny may have been originally derived from cornpone, the adjective, as in 'corpone humor' or 'what a cornpone idea'. (Cornpone, the noun, is a certain type of corn bread...right, Jackie? )

from the AHD:

cornpone

SYLLABICATION: corn·pone
PRONUNCIATION: kôrn pn
VARIANT FORMS: or corn pone
NOUN: Chiefly Southern & Midland U.S. See johnnycake. See Regional Notes at johnnycake, light bread, pone.
ADJECTIVE: Informal Folksy and homespun, as in manner or speech: a penchant for cornpone humor; cornpone political prose.

And a belated merry welcome, websafe, to our roving band of Wordable Herdables!