I also thought the longest word in the English language was antidisestablishmentarianism, but it is in fact pneumonoultramicroscopicsilicovolcanoconiosis – a lung disease caused by inhalation of silicate or quartz dust, apparently. AdvertisementEven as an adult, though, I would never have guessed what some experts deem the “most complex” word a standard dictionary has to offer. Why is “run” considered the most complex word in English? It’s because the word has so many definitions (645 for the verb form alone, the pros say). Speaking to NPR, Simon Winchester, author of The Professor and the Madman: A Tale of Murder, Insanity and the Making of the Oxford English Dictionary, said that in the first Oxford English Dictionary, “set” actually took up the most real estate with 200 definitions.