The antonyms for "light-hearted" are those that describe someone or something as serious, solemn, or melancholic. The term "heavy-hearted" can be used to describe someone who is feeling deeply sad or troubled. Other antonyms include solemn, gloomy and melancholic. If a person is characterized as "heavy-handed", it means that they are excessively serious or stern. "Sad" and "dreary" are other words often used as antonyms of light-hearted. These words are used to describe a person who lacks the joyful spirit, spontaneity, and lightheartedness that are often associated with the term "light-hearted.