cross one's heart
v. phr., informal 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.

[TahlilGaran] English Idioms Dictionary ▲