Cross one's heart is an idiomatic phrase that indicates that what someone is saying is true or that they promise to do something. Some antonyms of this phrase are "lie," "deceive," or "break a promise." When someone says "cross my heart," they are showing a symbol of honesty and sincerity. Therefore, when someone is lying, being dishonest or breaking their promise, they are not crossing their heart. Using the opposite of the phrase describes someone who is unreliable and is not to be trusted. So, when someone promises something, it is crucial to ensure that they are honest in their intention before trusting their commitment.
