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.
This is an expression for ‘the best’ or ‘something good’ that first makes its appearance (in American English) in the early twentieth century. There is no agreed etymology. Cassidy, in his book How The Irish Invented Slang, claims that it comes from the Irish deach, which means ‘the best’.
There are several problems with this. Firstly, the pronunciation is not at all close. Compare the sound files for teach (https://www.focloir.ie/en/dictionary/ei/house) to see how a word that rhymes with deach is pronounced. Secondly, deach is an archaic expression. It is marked in Ó Dónaill’s dictionary as Lit. – a literary expression. A look through Corpas na Gaeilge shows that there are no uses of this word in the corpus itself after the 17th century. (Later uses of deach, as in Cín Lae Amhlaoibh, are misspellings of other words, such as deich, the Irish for ten.) In modern Irish, there are other words meaning the best, such as ‘an ceann is fearr’ or ‘togha’. In other words, it is highly improbable that it would have been known or used by Irish speakers in 19th or early 20th century America.