Today's OED antedating: "to whistle in the dark"

To whistle in the dark, figurative, to defy hopelessness ("whistle," v., sec 9 (b), first use cited 1939): Untitled item by Franklin P. Adams, New York Tribune, July 23, 1915, p 7, col 1: "The notion of the impregnability of our great, in area, nation is that of whistling in the dark. It is like Princeton singing about 'rolling up the score' and 'fighting with a vim, for we're sure to win' when the score is about 28 to 0 in favor of Yale."