whistlin

What you just said is absolutely true; you ain't kidding. "Dixie" was a song popular among Confederate soldiers during the US Civil War, eventually coming to represent idle chitchat, silly fantasies, or foolish nonsense. Primarily heard in US. A: "That storm looks like it could do some serious damage." B: "You ain't just whistling Dixie! We'd better get all the farm equipment stowed inside before it hits." A: "Hoo boy, these financial results are pretty grim." B: "You ain't whistlin' Dixie, Tom."

Rur. You are right. Tom: Sure is hot today. Bill: Yeah, you ain't just whistlin' Dixie. It's a scorcher. Charlie: That was a good movie. Jane: You ain't just whistlin' Dixie. It was the best I've ever seen.

You said a mouthful. The origin of this expression has been lost, but it is generally thought to allude to the 1860 song “Dixie,” with words and music by Dan Emmett. Originally written for a minstrel show, it became famous as a Civil War marching song of the Confederacy, “Dixie” being a nickname for the South whose origin has also been lost. Allegedly General Pickett, just before he made his famous charge at Gettysburg, ordered that the song be played to bolster the morale of his troops. The saying presumably means that you’re not just whistling the marching song and mouthing empty words, but instead getting down to the actual combative meaning. See also you said a mouthful.