This article is from the the official alt.spleen FAQ, by Andrew Stellman with numerous contributions by others.
Andrew says: NOTE: PLEASE DO NOT SEND ANY MAIL TO ME REQUESTING MEDICAL ADVICE! I AM NOT A DOCTOR, AND WILL NOT GIVE YOU ANY USEFUL INFORMATION.
Etymologist Brian Youmans (email@example.com) came up with the following etymology. According to Brian:
"[the word 'spleen'] has very ancient roots, going back to the Middle English, 'splen', the Old French 'esplen', the Latin 'splen', the Greek 'splen' (long e there), and the Indo-European 'sphelgh', from which the Sanscrit 'plihan' (long i) and Old Slavic 'slezena'."