The word "bilious" has evolved from its literal medical meaning to a figurative description of mood. In earlier times, when humoral theory dominated medicine, it was believed that imbalances in the body's humors (such as bile) could lead to both physical ailments and emotional disturbances. In this context, a "bilious" person was thought to have a temperament characterized by irritability or anger.
Today, the term is rarely used in medical contexts but still appears in everyday language to describe someone who is unpleasant or quick-tempered. While the medical roots of "bilious" have faded, its use in describing mood continues to persist, often in a more metaphorical sense.