Triskaidekaphobia (is fear or avoidance of the number 13. It is also a reason for the fear of Friday the 13th, called paraskevidekatriaphobia (from Paraskevi, Greek for Friday) or friggatriskaidekaphobia (after Frigg, the Norse goddess after whom Friday is named in English). The term was used as early as in 1910 by Isador Coriat in Abnormal Psychology.

From the 1890s, a number of English language sources relate the “unlucky” thirteen to an idea that at the Last Supper, Judas, the disciple who betrayed Jesus, was the 13th to sit at the table. The Bible says nothing about the order in which the Apostles sat, but there were thirteen people at the table. Also, the number 13 is not uniformly bad in the Judeo-Christian tradition. For example, the attributes of God (also called the Thirteen Attributes of Mercy) are enumerated in the Torah (Exodus 34:6–7).

There is a myth that the earliest reference to thirteen being unlucky or evil is in the Babylonian Code of Hammurabi (circa 1780 BCE), where the thirteenth law is said to be omitted. In fact, the original Code of Hammurabi has no numeration. The translation by L.W. King (1910), edited by Richard Hooker, omitted one article:

If the seller have gone to (his) fate (i. e., have died), the purchaser shall recover damages in said case fivefold from the estate of the seller. Other translations of the Code of Hammurabi, for example the translation by Robert Francis Harper, include the 13th article.