Английский словарь американских идиом - cross one's heart

or cross one's heart and hope to die {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./