Definition from Wiktionary, the free dictionary
Refers to the song "Dixie", the traditional anthem of the Confederate States of America. The full implication is that the Confederacy would not succeed in the American Civil War through sentiment or token action alone.
- (idiomatic, southern US) To engage in idle conversational fantasies.
- He said he was going to open a business next year, but I think he was just whistling Dixie.
- "Sure is hot!" / "You ain't whistlin' Dixie!
Usage notes 
- Frequently used in the negative, to mean someone or something is serious, as in, When I say that, I'm not just whistling Dixie, I really mean it.
to engage in idle conversational phantasies