Origins
The word 'spleen' entered English in the thirteenth century from Old French 'esplen,' from Latin 'splēn,' from Greek 'splḗn' (σπλήν). The Greek word has possible cognates in Sanskrit 'plīhán-' (spleen) and Avestan 'spǝrǝzan' (spleen), but the deeper PIE etymology is uncertain. The native Germanic word for the organ is different: German 'Milz,' Dutch 'milt,' Old English 'milte' — English inherited both the Greek-Latin and Germanic terms, but the Greek form won out in standard use, while 'milt' survives in some dialects.
The spleen's dual identity — as both a physical organ and a metaphor for emotional states — comes from ancient humoral medicine. The Greek medical tradition, codified by Hippocrates and Galen, held that the body contained four humors (fluids) whose balance determined health and temperament: blood (from the heart), yellow bile (from the liver), phlegm (from the brain), and black bile (from the spleen). An excess of black bile — Greek 'melaina cholḗ' (μέλαινα χολή), which gave us 'melancholy' — was believed to cause depression, irritability, moroseness, and anger.
This humoral theory made the spleen the seat of the darkest emotions. To 'vent one's spleen' is to release pent-up anger. A 'splenetic' person is chronically bad-tempered. In Renaissance and early modern English, 'spleen' was used as a synonym for a wide range of negative emotions: melancholy, spite, caprice, mirth (paradoxically — the spleen was considered responsible for both depression and sudden fits of laughter), ill humor, and hypochondria. Shakespeare uses 'spleen' frequently in this emotional sense: in A Midsummer Night's Dream, Puck resolves to cause mischief 'in very spite of spleen.'
Development
The physical organ — a dark reddish-purple, fist-sized structure located in the upper left abdomen — had mysterious functions that were not understood until the nineteenth and twentieth centuries. Unlike the heart (which clearly pumps), the lungs (which clearly breathe), and the stomach (which clearly digests), the spleen offered no obvious function to early anatomists. This mystery contributed to the organ's association with occult humoral influences.
Modern medicine has revealed the spleen's actual functions, which are primarily immunological and hematological. The spleen filters blood, removing old and damaged red blood cells. It stores a reserve of blood that can be released during hemorrhage. It produces antibodies and plays a crucial role in the immune response, particularly against encapsulated bacteria. The spleen is also involved in recycling iron from destroyed red blood cells.
Despite its importance, the spleen is not essential to life. The body can survive without it (a condition called asplenia), though splenectomized patients face increased risk of certain infections and typically require vaccinations and sometimes prophylactic antibiotics. Splenectomy (surgical removal of the spleen) may be necessary after traumatic rupture, since the spleen's rich blood supply makes splenic bleeding life-threatening.
Greek Origins
The medical vocabulary built on 'spleen' includes 'splenic' (adjective), 'splenomegaly' (enlargement of the spleen, from Greek 'mégas,' large), 'splenectomy' (surgical removal), and 'asplenia' (absence of the spleen). 'Splenetic' has largely left medical vocabulary and survives primarily as a literary word for ill-tempered or peevish — a ghost of humoral medicine in modern English.