Fear of the number 13: A morbid fear of the number 13. Also known as triskaidekaphobia. Written reference to the superstitious fear of the number thirteen dates to the late 1800s. Its origin is conjectural (a matter of guesswork). The term triskaidekaphobia first appeared in the early 1900s. It was derived from treiskaideka, the Greek word for thirteen + phobia, fear of = a fear of thirteen.