cross my heart and hope to die
Attest to the truth of something; solemnly assure someone that the truth has been spoken. For example, I did lock the door—cross my heart and hope to die! This phrase most likely originated as a religious oath based on the sign of the cross; it is generally accompanied by hand gestures such as crossing one's hands over one's breast and then pointing the right hand skyward (a variant is cross my heart and point to God). Today most often uttered by children, it was first recorded in 1908.