The term paraskevidekatriaphobia was first coined in the early nineties by Dr. Donald E. Dossey, an American psychotherapist specialising in phobias and stress management, who reputedly claimed that when someone was able to pronounce the word they were cured. The term is based on the Greek words paraskevi ('Friday') and dekatria ('thirteen') with -phobia as a suffix to indicate 'fear'.
No-one is quite certain why people associate Friday the 13th with bad luck. Various theories have been put forward over the years, including Friday as the day of Christ's crucifixion and the ancient Egyptian belief that the 13th stage of life is death. Whatever the roots of this irrational fear, it seems to represent a very real concern to many people, so much so that a 1993 article in the British Medical Journal investigated the relation between 'health, behaviour and superstition surrounding Friday the 13th in the United Kingdom'.
The study reported the surprising finding that, despite the fact that fewer people chose to travel on Friday 13th, there were significantly more hospital admissions due to accidents than there were on 'normal' Fridays!