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.

An agreeable, trustworthy person. This slangy expression has outlived bad egg, which it actually implied in the sixteenth century. “Neither good egge nor good bird,” went the saying, meaning the young (egg) would not turn into praiseworthy adults (bird). In the nineteenth century this continued to be spelled out: “A bad egg [is] a fellow who has not proved to be as good as his promise” (The Athenaeum, 1864). The favorable aspect of good egg dates from the early twentieth century. Rudyard Kipling used it in Traffics and Discoveries (1904): “‘Good egg!’ quoth Moorshed.”

An amiable person. This slangy expression originated in America, probably in the late nineteenth century. It appeared in Meredith Nicholson’s Hoosier Chronicle of 1912: “Dad’s a good old scout.”