"the southern United States," 1859, of obscure origin, first attested in the song of that name, which was popularized, if not written, by Ohio-born U.S. minstrel musician and songwriter Dan Emmett (1815-1904); perhaps a reference to the Mason-Dixon Line, but there are other well-publicized theories dating to the Civil War. Popularized nationwide in minstrel shows. Dixieland style of jazz developed in New Orleans c. 1910, so called by 1919 (in the name of a band).
It is interesting to remember that the song which is essentially Southern — "Dixie" — and that which is essentially Northern — "Yankee Doodle" — never really had any serious words to them. [The Bookman, June 1910]