It might be "supercalifragilisticexpialidocious" (which appears in the Oxford English Dictionary), unless you want to count names of diseases (such as "pneumonoultramicroscopicsilicovolcanoconiosis," defined by the OED as 'a factitious word alleged to mean 'a lung disease caused by the inhalation of very fine silica dust, but occurring chiefly as an instance of a very long word'), places (such as "Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch," a village in Wales), chemical compounds (apparently there is one that is 1,913 letters long), and also a few words found only in Joyce's Finnegans Wake.
There y'all go.
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