Sesquipedalian

A long polysyllabic word that means having many syllables or using long words.

From the Latin sesqui- meaning one and a half times + ped, pes meaning foot. In Ars Poetica, the Roman poet Horace cautioned young poets against using “sesquipedalia verba” — “words a foot and a half long.”

For example, Pneumonoultramicroscopicsilicovolcanosconiosis.