The phrase cross my heart (and hope to die) is used to emphasise the truthfulness and sincerity of what one is saying, from the action of making a small sign of the cross over one’s heart, which sometimes accompanies the words.
It seems to have originated in the USA in the second half of the 19th century and to be first recorded in Fashions and Follies of Washington Life (Washington, D.C. – 1857), a play by Henry Clay Preuss. Capt. Jack Smith is an “Ex-Captain and retired politician—An old ‘fogy’”, and Tom Scott is “Capt. Smith’s body-servant—An ancient gentleman from Africa”:
– Tom. Here I is, Massy Jack.
– Capt. S. Have you obeyed all your master’s orders today, sir?
– Tom. Yes, Massy.
– Capt. S. Attended strictly to everything I told you?
– Tom. Yes indeed, Massy—cross my heart!
– Capt. S. Well done, thou good and faithful servant! take a drink, sir!
The earliest instance of cross my heart and hope to die that I could find is from The Compromiser, a short story by ‘E. P. H’, published in The Overland Monthly and Out West Magazine (San Francisco) of July 1892; this story begins with Frederick Dick, the son of the Justice of the Peace, saying the following:
“I’ve got two apples in my dinner-pail. Are you going home with me, Eva Wortley?”
The girl shook her yellow curls, and glanced toward Jim Parsons.
“I’ve got a secret what nobody knows, and nobody sha’n’t know, ’cause I won’t tell. Do you reckon you’ll go home with Jim Parsons, Eva?”
The diplomatist was soon trudging along the dusty road homeward bound from school, and by his side walked Eva Wortley. What if she thought him the laziest, dirtiest boy in her class; surely his secret was not grimy, too, and Jim would wait for her tomorrow.
“Now cross your heart, and hope to die you won’t let on ter [sic] living soul!” so the revelation was prefaced.
“Cross my heart, and hope to die.”
“You won’t breathe a word to Jim Parsons?”
“Hope to die.”
“Well, when I’m a man I am a going to be judge of that thar [sic] high court in Washington.”