spleen: [13] Spleen comes via Old French esplen and Latin splēn from Greek splén, which may have been related to Latin liēn ‘spleen’ and Greek splágkhnon ‘entrails’ (source of English splanchnic ‘of the viscera’ [17]). In medieval physiology many internal organs were held to be the seat of a particular emotion, and the spleen was no exception. It had several conflicting states of mind attributed to it, but the one which survives is ‘moroseness’ or ‘bad temper’, in the derived adjective splenetic [16].
spleen (n.)
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).