"Crosses heart" is a statement often used to show sincerity or honesty. However, some antonyms for this phrase could be "telling a lie," "being insincere," "breaking a promise," or "betraying trust." These words imply a lack of honesty or commitment, indicating that the person who is not crossing their heart may not be trustworthy or genuine. In contrast, synonyms for "crosses heart" include "swears by," "pledges," and "vows," all of which suggest a strong commitment to honesty and a sincere desire to keep promises. Ultimately, the choice of antonyms will depend on the context of the situation and the speaker's intention.