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.
The expression Holy Mackerel dates back to 1803. Holy Mackerel, as we’ve said before, belongs to a class of exclamations called minced oaths, where a similar word is said in order to avoid a vulgar or blasphemous term. Thus, the French say Sacré Bleu (Holy Blue) to avoid saying Sacré Dieu (Holy God), and the Irish say Dar Fia (by the deer) instead of Dar Dia (by God). Holy Mackerel is probably a minced oath for ‘Holy Mary’. Mackerel is particularly appropriate because the mackerel is associated with Roman Catholics – people of the Catholic tradition tend to eat fish on a Friday instead of meat, and mackerel was a common choice. Mackerelism was used as a pejorative slang term for Catholicism in the 19th century.
Daniel Cassidy claimed that this was wrong and that it derives from an Irish phrase mac ríúil – ‘kingly son’. In other words, it was supposedly something to do with Jesus. The problem is that while mac rí (son of a king) is a common phrase for a prince in Irish, mac ríúil is not a genuine Irish phrase. By definition, a prince is a mac rí. But princes are princely, not kingly and ríúil means kingly, not royal. That’s another word, ríoga. There is no evidence for mac ríúil (or the older spelling mac righiúil) existing in the Irish language as a term for a prince, or for Jesus, or indeed, for anything else. It is completely fake.