you ain't (just) whistling Dixie

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."
See also: Dixie, whistle
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

You ain't just whistlin' Dixie.

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.
See also: Dixie, just
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.

whistlin' Dixie, you ain't just

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.
See also: just
The Dictionary of Clichés by Christine Ammer Copyright © 2013 by Christine Ammer
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References in classic literature ?
I'n been wi' you twenty year, an' you can't get twenty year wi' whistlin' for 'em, no more nor you can make the trees grow: you mun wait till God A'mighty sends 'em.
"There doesn't seem to be no need for no one to be contrary when there's flowers an' such like, an' such lots o' friendly wild things runnin' about makin' homes for themselves, or buildin' nests an' singin' an' whistlin', does there?"
Why, I'm just whistlin' an' happy all day long, thinkin' of the boy an' seein' you at home here workin' away on all them nice things.
At the Whistlin Kirk near Glasgow's High Court, owner Annemarie Lynch said: "If this goes ahead I fear I'll have to close the pub and five jobs will be lost.
Plante also owns Whistlin Dixie Baseball, a Georgia-based consulting and player development firm.
"Nah, I was too busy dancing around the tent doing them pirouette things to do any whistlin,'" I said.