Mirthlessly is an adverb used to describe something done without joy or laughter. There are several synonyms that can be used for the word mirthlessly. These include solemnly, gravely, seriously, somberly, joylessly, cheerlessly, bleakly, and drily. All of these words suggest a lack of humor or lightness. They convey a sense of sadness or seriousness, and often evoke a feeling of gloom. When writing, it is important to choose the right word to accurately convey the tone and mood of the text. The use of synonyms can help to add variety and depth to the language, and is an important tool for any writer.