Obsolete A whim; a caprice. A large, highly vascular lymphoid organ, lying in the human body to the left of the stomach below the diaphragm, serving to store blood, disintegrate old blood cells, filter foreign substances from the blood, and produce lymphocytes. A homologous organ or tissue in other vertebrates. Obsolete This organ conceived as the seat of emotions or passions. Ill temper: vent one's spleen. Archaic Melancholy.