A Southern expression which can roughly be taken as a form of agreement with something that has been said. It can be translated as "no kidding", "that's the truth", and so forth. Alternatively, it can be added to emphasize that what someone is saying is important and should not be ignored. Here's a few quick examples, courtesy of Jethro Bodine:
Scenario 1: Person A has just won the lottery.
Person A: I ain't gonna eat store-brand macaroni tonight.
Person B: You ain't just whistling Dixie.
Scenario 2: Person A has just witnessed a barfight and saw someone have a 1.75 L bottle of tequila smashed across his face.
Person A: That boy got hurt bad.
Person B: You ain't just whistlin' Dixie.
And here's an example from EdSausage:
Scenario 3: Person A is giving instructions.
Person A: Always use a wooden stick to make a hole in the C4 for your blasting cap! I'm not up here whistling Dixie. Remember this, or you may blow yourself up.
Of course, the "Dixie" part of the expression refers to the song Dixie Land.
Strangely, I have been unable to locate the origins of this phrase on the Internet. In fact, I've run across three different origins of the word Dixie. So it appears there's some confusion about where this phrase came. I am, however, keeping my eyes open for new information.