Fear of the number 13
It's impossible to say just how or when the number thirteen got its bad reputation. There are a number of theories, of course. Some say it comes from the Last Supper because Jesus was betrayed afterwards by one among the thirteen present. Others trace the source of the superstition back to ancient Hindu beliefs or Norse mythology. But if written references are any indication, the phenomenon is fairly modern. Known mention of fear of thirteen in print dates back only to the late 1800s. By the early 20th century, however, it was prevalent enough to merit a name, which was formed by attaching the Greek word for "thirteen"—treiskaideka (dropping that first e)—to the Latin (and ultimately Greek-derived) form -phobia ("fear of"). The first known reference to triskaidekaphobia in print comes from a 1908 book by Isador Coriat, Religion and Medicine: the Moral Control of Nervous Disorders.
There are several odd facts about triskaidekaphobia: it's a word formed by combining two different languages (Greek and Latin); it's one of the top lookups in our online dictionary every time Friday the 13th rolls around; and though it's one of the Internet's favorite words, people tend to be much more interested in the word than in the phobia it describes. It's rare to see a list of long and interesting words that doesn't include it.
Trend Watch tracks popular lookups to see what people are talking about. You can always see all Trend Watch articles here.