Spleen
Middle English: shortening of Old French esplen, via Latin from Greek splēn ; spleen (sense 2) derives from the earlier belief that the spleen was the seat of bad temper.
wiktionary
From 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.
etymonline
spleen (n.)
c. 1300, non-glandular organ of the abdomen, from Old French esplen, from Latin splen, from Greek splen "the milt, spleen," from PIE *spelgh- "spleen, milt" (source also of Sanskrit plihan-, Avestan sperezan, Armenian p'aicaln, Latin lien, Old Church Slavonic slezena, Lithuanian blužnis, Old Prussian blusne, Old Irish selg "spleen").
Regarded in old medicine 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).
