'splenetic' originates from Late Latin 'spleneticus', ultimately from Greek 'splēnētikos', where 'splḗn' meant 'spleen'.

'splēnētikos' (Greek) became Late Latin 'spleneticus', passed into Middle English as 'splenetik' and developed into modern English 'splenetic'.

Initially it meant 'pertaining to the spleen', but over time it acquired the figurative meaning 'irritable, spiteful' and is now chiefly used in that sense.