"Spleen" refers anatomically to the abdominal organ but historically was considered the seat of various emotions and temperaments. Medieval humoral theory associated spleen with melancholy and black bile, making "the spleen" synonym for depression, ill humor, or anger. Renaissance poetry extensively explored spleen as emotional source—to "vent one's spleen" meant expressing anger, "suffering from spleen" meant melancholic. The term also meant caprice, spite, or sudden impulse. Poets valued its compact expression of complex emotional states rooted in physical theory. Shakespeare and contemporaries used it when depicting characters' temperamental conditions where humoral psychology explained personality and mood. The word's anatomical precision combined with emotional associations made it rich for exploring mind-body connections. Its use reflected historical beliefs about physiology's role in psychology, where organs generated emotions. The term appears in contexts of anger, melancholy, or capricious behavior where archaic physiological vocabulary provided concrete imagery for abstract emotional states. Its continuing metaphorical usage preserves obsolete medical theory in linguistic fossil form.

In early modern thought, the “spleen” was considered a centre of emotion, particularly linked to melancholy or ill temper. Poets of the seventeenth and eighteenth centuries often invoked it to personify moodiness, passion, or caprice. Knowing this helps modern readers grasp metaphors where “spleen” conveys emotional turbulence rather than the literal organ.