Why is the ninth month called September?
c.1300, from Old French esplen, from Latin splen, from Greek splen, from PIE *splegh- (cf. Sanskrit plihan-, Avestan sperezan, Armenian p'aicaln, Latin lien, Old Church Slavonic slezena, Lithuanian bluznis, Old Prussian blusne, Old Irish selg "spleen"). Regarded in medieval physiology as the seat of morose feelings and bad temper. Hence figurative sense of "violent ill-temper" (1590s).
A large, highly vascular lymphoid organ, lying to the left of the stomach below the diaphragm and serving to store blood, disintegrate old blood cells, filter foreign substances from the blood, and to produce lymphocytes.
An organ in vertebrate animals that in humans is located on the left side of the abdomen near the stomach. The spleen is mainly composed of lymph nodes and blood vessels. It filters the blood, stores red blood cells (erythrocytes) and destroys old ones, and produces white blood cells (lymphocytes).