cross one's heart

To say that what you have said is surely true; promise seriously that it is true. — Often used by children in the longer form. Children often make a sign of a cross over the heart as they say it, for emphasis.
"Cross my heart, I didn't hide your bicycle," Harry told Tom.
"I didn't tell the teacher what you said. Cross my heart and hope to die," Mary said to Lucy.
Categories: children used by children {informal} {v. phr.}