spleen (n.) Look up spleen at Dictionary.com
c.1300, from Old French esplen, from Latin splen, from Greek splen "the milt, spleen," from PIE *spelgh- "spleen, milt" (cognates: Sanskrit plihan-, Avestan sperezan, Armenian p'aicaln, Latin lien, Old Church Slavonic slezena, Lithuanian blužnis, 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" (1580s, implied in spleenful); and thence spleenless "free from anger, ill-humor, malice, or spite" (1610s).