What I’m saying is really true. Originally a solemn oath for veracity, this phrase became a schoolyard assertion. The first version was traditionally accompanied by crossing one’s arms over the chest and then raising the right arm. The cliché dates from the second half of the 1800s. A twentieth-century synonym is Scout’s honor!, alluding to the promise of honesty taken by Boy (and Girl) Scouts. It dates from about 1900. J. A. Jance had it in her mystery novel, Devil’s Claw (2000), “Joanna was shocked. ‘You didn’t tell her that!’—Now it was Butch’s turn to grin. ‘I did,’ he said. ‘Scout’s honor.’” And Jan Burke even combined the two: “‘Swear you’ll keep me posted on your progress?’—‘Girl Scout’s honor. Or may I simply cross my heart?’” (Remember Me, Irene, 1996). Also see honest to goodness.