It is actually a humorous exaggeration created by the author Quintus Horace Flaccus as sesquipedal ([words of] foot and a half [long]") attached to the suffix -phobia to invent a name for the "fear of long words". It is one of the best of all those that appeared (see list of synonyms), only because it makes it clear that it is not serious, and is very, very nardo. It became famous as part of the hypopotomonstrosesquipedaliophobia hoax.
