What is "cross my heart and hope to die"

phr. (context idiomatic English) (non-gloss definition: added to a promise or a statement to show how serious one was about being honest in what one said)

Cross my Heart and Hope to Die is a 1994 Norwegian film directed by Marius Holst. It is loosely based on Lars Saabye Christensen's novel Gutten som ville være en av gutta. The film was selected as the Norwegian entry for the Best Foreign Language Film at the 67th Academy Awards, but was not accepted as a nominee.