Unplayful is a word that suggests a lack of frivolity, seriousness, or lightheartedness. Some synonyms for the word unplayful include serious, solemn, somber, grave, austere, staid, or earnest. These words all convey a sense of gravity or weightiness and indicate that the situation or person is not meant for levity or playfulness. Unplayful can also suggest that someone is not playful or humorous, which can be described using synonyms like humorless, unamusing, dour, or glum. Ultimately, the choice of synonym for unplayful will depend on the context and the intended meaning, but all of these words capture the essence of a lack of playfulness.