spleen

In a biological sense, this refers to the lymphatic organ responsible for filtering blood. It is a standard countable noun in medical contexts. When used to describe emotion, it refers to a deep-seated bitterness or spite. This usage stems from the archaic medical belief that the organ was the source of melancholy and anger. In this figurative sense, it is often treated as an uncountable mass noun, particularly in the idiom "vent one's spleen," which describes the act of releasing pent-up rage.