Well, no, it ain't.
The currently accredited longest word in English is pneumonoultramicroscopicsilicovulcanoconiosis. This is a horrible dust disease of the lungs caused by exposure to volcanic debris. Often goes unnoticed, as entombment and mummification will follow in short order if you're too close to that old crater.
The internet proclaims it to be the longest in English, which is the first whiff that it's bullshit. Secondly, it's not bloody English, it's mainly Greek with bits of Latin thrown in for reinforcement. Like a very fine Pompeiian terrazzo floor.
The former contender, floccinaucinihilipilification, admittedly also suffered this problem, as do many English words. It's all very well for words meaning horrible diseases to be foreign or ancient, because ancient foreigners taught us all that stuff, but it's my view that in an age of resurgent nationalism English ought to disallow for top honours any made-up, foreign, bloody silly, gigantic portmanteau words. For gods' sake, we're not speaking German or Welsh!
That leaves antidisestablishmentarianism, and it's a solid choice which weds the English to their God, Queen, and Country.
FLUMMOXED BY THE FICKLENESS OF ENGLISH
PETER THE PROOFER IS HERE!