The word triskaidekaphobia is derived from Greek triskaideka ‘thirteen’ and phobos ‘fear’. The word therefore means fear of the number 13. The fear of Friday the 13th can be said to have been originating from this condition (called paraskevidekatriaphobia—from Greek paraskevi ‘Friday’ or friggatriskaidekaphobia—from Frigga, the Norse goddess who is the namesake of the English word Friday). The word has been recorded since at least 1910 where it was found in Abnormal Psychology by Isador Coriat. There have been two theories explaining the origin of the phobia.
The Judas theory tries to explain that the fear stems from the fact that Judas was the 13th disciple of Jesus to sit at the table during the Last Supper and he was the one who betrayed the latter. This theory began in the 1890s. The Bible itself made no mentions of the order in which the Apostles sat but there were indeed 13 people around the event. The Hammurabi theory tries to explain that a reference to unlucky number 13 appeared as early as 1780 BCE in the Babylonian Code of Hammurabi. The Code omitted the 13th law.
There are events that people from associated with the number 13. Apollo 13 suffered an explosion on its oxygen tank on April 13, 1970. The Knights Templar was arrested on Friday, October 13, 1307 by the order of Philip IV of France. At the time, only the number 13 was considered unlucky; there was no fear of Friday the 13th. The idea that their arrest was related to phobia of Friday the 13th is incorrect and was popularized by the novel The Da Vinci Code.
Fear of specific number, however, does not only apply to the number 13. There is hexakosioihexekontahexaphobia (fear of number 666). Tetraphobia is common in Vietnam, Korea, Japan, Singapore, Taiwan, and China, among others. In Italy, number 17 is considered unlucky. The Roman numeral for 17 is XVII, which when rearranged to VIXI would mean “I have lived”. This, in turn, is considered a euphemism for “I am dead”. A number of western cultures regard Friday the 13th to be unlucky—hence the aforementioned paraskevidekatriaphobia. Tuesday the 13th, however, is considered equally unlucky in Greece and some parts of Spain and Latin America. In some parts of Afghanistan, there is something called the curse of 39. The number 39 is thrice thirteen. It is considered cursed or a source of shame.