A hapax legomenon is a word that occurs only once in a given body of text:

• NORTELRYE (“education”) was used only by Chaucer, and only once.
• AUTOGUOS, an ancient Greek word for “plow,” was used only once, in Hesiod.
• FLOTHER, a charming synonym for “snowflake,” appears only once in written English before 1900 (in a manuscript from around 1275).
• PIM, a stone weight of about a quarter ounce, appears only once in Biblical Hebrew (1 Samuel 13:20).

HONORIFICABILITUDINITATIBUS appears only once in Shakespeare’s works (in Love’s Labour’s Lost). Probably a good thing.