Sorry, Webster 1913, but you're wrong about the etymology of rankle. It's much more interesting than just being the verb from the adjective rank. Current wisdom is it in fact comes from the Old French drancler, with a D that dropped away. This came from the Latin dracunculus, a little dragon, and referred to an ulcer or tetter or pain that gnawed away at you like a little dragon. See also dracunculiasis for a modern disease with the same metaphor.

Here's the complete entry from the Chambers Dictionary:

rankle, rangk'l, v.i. to fester : to cause festering : to go on vexing, irritating, or embittering.--v.t. to cause to fester : to envenom : to embitter.--n. a rankling. [O.Fr. rancler, raoncler--draoncler, app.--L.L. dra(cu)nculus, an ulcer, dim. of L. dracō--Gr. drakōn, dragon.]
And here it is in the unrelated Pocket Oxford Dictionary:
rankle (răng'kl), v.i. (Of envy, disappointment, criticism, &c.) gnaw at the heart, cause recurrent pangs. [F rancler, drancler, f. L dracunculus little serpent]