Cross one’s heart (and hope to die)
There is a strong feeling that the habit of crossing one’s heart goes a long way back in time and is connected to the old religious practice of making the sign of the cross as proof that one is telling the truth. There is no citation, however, that proves this. Cross one’s heart and hope to die is first cited from the early 20th century as a children’s oath emphasising that one is telling the truth. Why it should suddenly materialise in the 20th century has never been explained.