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.