Yup, that’s a real word, which I just learned today. In case it’s not part of your daily vocabulary, here’s what it means: a fear of peanut butter sticking to the roof of one's mouth. Etymology –arachi (Greek) means ground nut, butyro (Latin) means butter, and phobia (Greek) means fear. This is a good example of what I was taught in my high school course on Latin and Greek Derivatives. Namely, that much of the English language has been borrowed from Greek and Latin, and it didn’t seem to bother anybody to combine roots from both to form a word that, at the time, seemed like a good idea. Two other real words that one might not have used in the past several weeks: thanatognomonic and aggiornamento.
Arachibutyrophobia, thanatognomonic, and aggiornamento . . . genuine words that an author of fiction would likely do well not to use unless an end-of-book glossary is included.
My point is that there is a wonderful world of words, some of which would not add much to – or even detract from – a good story. A varied and colorful vocabulary is to be treasured and used, but discretion is encouraged. Avid readers generally like to be challenged with new words; it can enhance the tale and be a learning experience. The caution is to not overdue it, especially using little-used, unfamiliar words which come across as the writer attempting to appear erudite. If a good word fits appropriately in the description or dialogue, by all means use it. Applied prudence in the use of vocabulary is part of the writing exercise.