Writinghood author Chang Lee Peng has a list of what he considers to be the 10 most difficult words to spell in the English language, such as Floccinaucihihilipilification, which means "act or habit to deny the value of some particular things.”
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a certain polygon (composed of triangles squares and pentagons) that has about 200 faces
rawm-ba-trun-kate-ed-eye-co-si-do-de-kuh-he-dron
(13 syllables)