cross one's heart
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.
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Source: A Dictionary of American Idioms