Hippopotomonstrosesquipedaliophobia

From hippopotomonstrosesquipedalian, an extension of sesquipedalian with monstrum “monster” and a truncated, misspelled form of hippopotamus, intended to exaggerate the length of the word itself and the idea of the size of the words being feared; combined with phobia.

– source

In other words? It is the fear of long words.