Mercurial/mɜːrˈkjʊriəl/ Definition 1 of 4Pro Tip 1/3Emotional Implication Suggests someone's mood is unpredictable, which might be viewed negatively. The coach's mercurial decisions make the team anxious. Pro Tip 2/3Not Literal 'Mercurial' describes behavior, not physical changes or appearances. His mercurial stance on the project kept everyone guessing. Pro Tip 3/3Use with Caution Can be insulting if used to describe someone without careful thought. Being labeled mercurial affected her reputation at work.