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.