Meaning

noun a large dark-red oval organ on the left side of the body between the stomach and the diaphragm; produces cells involved in immune responses

noun a feeling of resentful anger

originFrom Middle English splene, splen, from Anglo-Norman espleen and Old French esplein, esplen, from Latin splēn (“milt”), from Ancient Greek σπλήν (splḗn, “the spleen”). Doublet of lien. Partially displaced the native English term milt.

Etymology from Wiktionary, CC BY-SA 3.0