Cross my heart.
This idiom or expression has to do with:
Creating trust.
Assuring the listener that the statement is true.
Q. Can I be sure you’ll be there?
A. I’ll be there. I promise. Cross my heart!
History:
The complete idiom is: Cross my heart and hope to die. It makes a reference to the sign of the cross. It was, and still is, believed that by crossing your heart you will keep the devil away.