A Dublin doctor who wants to be anonymous wrote to ask about the verb rankle. He tells me that in an 18th century medical tract he picked up in London recently rankle was equated with fester. Elaborate, he commands.
The modern senses of the verb rankle, to cause severe and continuous irritation, anger or bitterness, are figurative extensions of the Middle English verb ranklen, which indeed means to fester, so the 18th century tract is correct. Rankle came into English from the Middle French rancler, which in turn came from the Old French noun rancle, a festering sore, which had quite a few variants, draoncle and drancle among them. The d gives a clue to the noun's origin: the Latin dracunculus, which is the diminutive of draco, a little snake or dragon. In medieval Latin dracunculus was a physician's name for a cancerous tumour, because it looked like a little dragon; similarly, rancle, a festering sore, looked like a little dragon or serpent to medieval French doctors.
We'd be at a loss to explain how dracunculus eventually gave us rankle but for the great 17th century lexicographer Du Change, who, as he was reading medieval medical texts, kept coming across the Late Latin contraction dranculus, which is close enough to both the French and English.
A similar case, Webster points out, is the keloid tumour, from French keloid, named from the Greek chele, claw, because of its shape.
Fester is from the Middle English noun fester, a suppurating sore. It comes through Middle French from the Latin fistula, a pipe or tube. Medieval physicians used the word for an abnormal opening, caused by ulceration, between one hollow organ and another or between a hollow organ and the surface of the skin, a description acceptable to modern pathologists, too, I'm told.
By the way, fistula was also an old name for a wind instrument. The story is told of the rather pedantic Celtic scholar Myles Dillon asking a bemused Wexford travelling piper, Johnny Doran, to explain the mechanics of his fistula to him in a pub in Ring during the war.
Finally, shyster; Mary O'Sullivan from Limerick asks about its origin. Webster's American dictionary, the last word on American English, admits that this word has led them a merry dance. They have now concluded that it is probably an alteration of an earlier shicer, a contemptible fellow, from German scheisser, to defecate.