The antonyms for the word "trueheartedness" are insincerity, disloyalty, betrayal, deceit, dishonesty, cunning, and treachery. Insincerity refers to lack of truthfulness and genuineness in words and actions. Disloyalty denotes betrayal and failure to uphold honesty and faithfulness in a relationship. Betrayal suggests breach of trust and faith. Deceit is the act of being dishonest and deceitful to achieve one's selfish motives. Dishonesty refers to the lack of truthfulness, and cunningness represents artfulness in deceiving others. Treachery describes disloyalty and betrayal committed with an ulterior motive. All these antonyms of "trueheartedness" denote lack of sincerity, honesty, and loyalty in a person's character.