“Dost thou know me bladder, / Thou insolent impostume?” snarled a character in John Fletcher’s The Island Princess. That was in 1621, when people had a more imaginative way with insults. Impostume is now rare, its infrequent escapes from the less-thumbed pages of our dictionaries being mostly in quotations from old herbals.
That’s because an imposthume or impostume is an abscess. It’s from Greek via the Latin apostēma. The Oxford English Dictionary notes that it’s “a word which has undergone unusual corruption”. On its way to us through French it was successively modified to empostume and then impostume. Meanwhile, Middle English had apostume, taken directly from Latin. This lost its initial vowel by a process called aphesis to become postume. By confusion with humus, an h was inserted to make posthume (the same thing happened with posthumous, from Latin postumus). By analogy, people came to believe impostume should similarly be spelled imposthume, the most common form from about 1700.
By the seventeenth century, impost(h)ume had become figurative, meaning a state of moral corruption, a festering sore on the body politic, or somebody metaphorically swollen with pride. This last sense was the way John Fletcher meant it. Much later, another writer applied it to our language:
Studied obscurity of thought and language, verbal finicalities and conceits, and mere ingenuities of any kind, rhythmic, mental or sentimental, will not meet the occasion: that sort of thing is overdone already. It is the “swollen imposthume” of refinement, an excrescence on culture, a penalty of which we have suffered enough.
Lippincott’s Magazine (Philadelphia), August 1878. A finicality is something finicky or fussy.