Capricious mood refers to a sudden and unpredictable change in someone's emotions or behavior. There are several synonyms for this phrase, including erratic, volatile, unpredictable, whimsical, and fickle. Erratic implies a lack of consistency or reliability, while volatile suggests extreme instability. Unpredictable implies a lack of foresight or pattern, while whimsical suggests a tendency to act on impulse or fancy. Fickle implies a lack of constancy and a tendency to change one's mind frequently. All of these synonyms convey the idea of mood swings and an inability to predict or control one's emotions, which can be challenging for those who are affected by them.