definition from
Oxford Dictionary of English – https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.dictionaryofenglish

spleen /spliːn /
▸ noun
1 Anatomy an abdominal organ involved in the production and removal of blood cells in most vertebrates and forming part of the immune system.
2 [mass noun] bad temper; spite:
he could vent his spleen on the institutions which had duped him.
spleenful /ˈspliːnfʊl / adjective
– ORIGIN 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.