Hapax legomenon is
Greek for "said only once". It is commonly used for words and phrases that appear only once in the history of recorded work. These phrases are often
idiomatic, and are frustrating to translate, as they have no other contexts from which to
glark their meaning.
My father once wrote a short story called "Hapax Legomenon", which was about a ghost that was cursed to haunt the classics library at Oxford looking for a word that he had come across during his lifetime.