For some time now, some of my on-line friends have advised me to provide a version of CassidySlangScam without the invective aimed at Cassidy and his supporters. In response to that advice, I am working on providing a glossary of the terms in Cassidy’s ludicrous book How The Irish Invented Slang with a short, simple and business-like explanation of why Cassidy’s version is wrong.
Holy Cow is a ‘minced oath’, a way of avoiding a blasphemous or offensive expression by using a similar word, or a word beginning with the same sound. This is thought to be a version of ‘Holy Christ’, but was probably influenced by the sacredness of cows in the Hindu tradition.
To Cassidy, it represents a mixed Irish and English oath, Holy Cathú. (Originally, he had claimed that the Holy represents the Irish oille meaning greatness but he had dropped this claim by the time the book was published.) Cathú usually means temptation in modern Irish, though it has other meanings like rebellion, grief, fighting. I presume the meaning of temptation came about through the idea of rebellion against God, as the root of the word is cath, meaning battle.
Cathú is pronounced kahoo. It is not used as an exclamation in Irish. People do not say ‘Cathú Naofa’ in the Irish language. Cassidy once again demonstrates his lack of Irish by miscopying the phrase ‘Mo chathú é’ from Dinneen’s dictionary as ‘Mo cathú é’. This phrase seems to exist but is only found in one source from 1909, so it is hardly a common expression.