the word "spleen" entered english in the early 14th century from the old french "esplen," which itself derived from the latin "splēn." latin borrowed the term from the ancient greek "σπλήν" (splḗn). the greek term is believed to have a proto-indo-european origin, possibly from a root meaning "to be full" or "to swell," which is a fitting description of the organ's physical appearance. in ancient and medieval medicine, the spleen was considered the seat of emotions like melancholy, ill humor, and anger, a belief that persisted into early modern english. this physiological association directly gave rise to the modern figurative meaning, where "spleen" denotes feelings of irritability or spite.

the word is a simplex loanword from latin via old french. it cannot be meaningfully segmented into productive morphemes in modern english.