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.
Cassidy points out that expressions like daid and daidí and daidín in Irish resemble the English words dad and daddy.
There is no doubt that the words dad and daddy go back a long way in English. Dad is found as far back as 1500 but is probably much older. Some sources have claimed that this is of Celtic origin, and this is the view that Cassidy takes (though Cassidy was not trained as a linguist, and his input in this case is completely uninformed and therefore irrelevant).
Douglas Harper’s excellent Online Etymology Dictionary takes a different view, pointing out that such forms (like mum, mammy and mama) are ‘nearly universal and probably prehistoric’ (https://www.etymonline.com/word/dad).
Whatever the origins of dad and daddy, Cassidy was not the first to make a claim of Celtic origins for these words, so there is nothing original in his treatment of this subject.