Yesterday, I thought I'd scored a bit of a coup by finding the earliest known use of jerk to mean a stupid person. But Ben Zimmer posted it to the American Dialect Society mailing list, where Doug Wilson suggests it's Turk, not jerk. Listen here and tell me what you think.
I can't find an authoritative source for the lyrics. The web certainly prefers jerk to Turk by about a factor of 10 to 1. But there's the book, Nowhere in America: the Big Rock Candy Mountain and other comic utopias, which claims to use the lyrics by permission of McClintock's estate, and it has Turk.
[Update, March 9: Royalty Recovery Inc negotiated the royalties for McClintock's estate for the O Brother album. Sadly, Jeff Gandel from Royalty Recovery confirms that the original lyric is Turk. Oh well...]