Falseheartedness is defined as the act of being insincere or disloyal. There are several synonyms for this word, including duplicity, deceitfulness, and treachery. Duplicity refers to the act of being deceitful or dishonest by presenting a false appearance or behavior. Deceitfulness is similar, but it emphasizes the intent to mislead or deceive. Treachery, on the other hand, refers to the act of betraying someone's trust. Other possible synonyms for falseheartedness include insincerity, hypocrisy, and betrayal. Regardless of the exact word used, falseheartedness is a negative trait that can damage relationships and erode trust over time.