Triskaidekaphobia is the fear of the number thirteen. It began life, and has pretty much remained, a psychological jargon term. Its first appearance in English is in Isador Coriat’s 1911 Abnormal Psychology.
Like many scientific terms, this one is a modern invention based on Greek roots. Triskaideka, or τρεισκαιδεκα, is Greek for thirteen, to which the suffix -phobia, or -φοβία, meaning fear, is added.
(Source: Oxford English Dictionary, 2nd Edition)
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