You ain't just whistling Dixie, and that's the truth! Whistling Dixie, which refers to a studied carelessness, comes from the song that originated in minstrel shows and from which the South takes its nickname. But if you say someone ain't just whistling Dixie, it means they're not kidding around. This is part of a complete episode.
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Grant mentions how a woman whistling Dixie might somehow not be a good thing (according to people back in the day). I grew up in Texas and heard the phrase "Whistlin' women and crowin' hens, will never come to no good end." Of course my mom (the only one I heard this from) has a penchant for these great old sayings and only used it jokingly, never with actual disapproval!
My SW Missouri grandmother had a slightly different version, "Whistling girls and crowing hens, often come to some bad end."
Emmett