The antonyms for the word "true-hearted" or sincere, honest, and genuine are insincere, dishonest, and fake. Insincere people are those who lack honesty or genuineness in their actions or stance towards others. They might not say what they truly feel or mean and pretend to be someone they are not. Similarly, dishonest people are those who often deceive or mislead others for their personal gain or advantage. Lastly, fake people are those who might portray or mimic the qualities of being true-hearted or honest but, in reality, lack authenticity and sincerity in their character. All these antonyms have one thing in common, which is the lack of honesty and genuineness in their personality.