The phrase "crossed one heart" implies making a sincere promise or taking an oath. Antonyms for this phrase would be dishonesty, insincerity, and lying. Someone who is not truthful or does not keep their promises could be described as false, untrustworthy, or deceitful. Betrayal could also be considered an antonym, as it goes against the spirit of keeping one's word. Additionally, someone who is unreliable or flaky could be considered the opposite of someone who has "crossed their heart." While a promise is an affirmative action, breaking a promise or being untrustworthy is a negative one.