Metaphorically, the spleen is the organ associated with bad temper.
While no organ literally causes bad temper (which is related to complex neurological and psychological processes), the historical association of the spleen with anger and irritability is significant. In medieval times, it was believed that the spleen produced black bile, one of the four humors that were thought to influence temperament. An excess of black bile was believed to cause melancholy, irascibility, and a generally bad temper.
Therefore, even though modern science has debunked the humorism theory, the association of the spleen with a bad temper remains embedded in language and culture. When someone is described as "venting their spleen," it means they are expressing anger or ill-humor. It is important to note, however, that anger and other emotions are primarily regulated by the brain, specifically regions like the amygdala and prefrontal cortex, which play key roles in processing and controlling emotions.
