back-formation from aecidium "a cup-shaped or spheroidal aecium," borrowed from New Latin, irregularly for *oecidium, borrowed from Greek oikídion, diminutive of oîkos "house, abode, room" — more at vicinity, -idium
Word introduced as a substitute for aecidium by the Purdue University plant pathologist J. C. Arthur (1850-1942) in an effort to reform terminology for rust fungi; see "Terminology of the Spore-Structures in the Uredinales," Botanical Gazette, vol. 39 (Mar., 1905), pp. 219-22. Arthur in the same article introduced aecial and aeciospore. The earlier word aecidium was coined as a genus name by the author and botanist John Hill, in A General Natural History, or New and Accurate Descriptions of the Animals, Vegetables, and Minerals, of the Different Parts of the World, vol. II, A History of Plants (London: Printed for Thomas Osborne, 1751). Hill states on p. 64: "We have called this genus, distinguished by its peculiar cells, Æcidium, from the Greek οικιδιον, cellula." Since the 19th century dictionaries and mycological references have assumed on no grounds other than appearance that the word was borrowed from Greek aikía, "maltreatment, outrage, torture." (See, for example, Miguel Ulloa and Richard T. Hanlin, Illustrated Dictionary of Mycology [St. Paul: American Phytopathological Society, 2000], where aikía is incorrectly glossed "wound, lesion.") Hill might have avoided misinterpretation by transliterating the word oecidium, but given the identity of <ae> and <oe> in English pronunciation, the distinction may have seemed trifling.