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.
According to the late Daniel Cassidy, jag in the sense of being drunk (to have or get a jag on) is not related to the word jag meaning a load. According to him, it comes from the Irish word deoch, meaning drink (see here for pronunciation: https://www.focloir.ie/en/dictionary/ei/beverage#beverage__2)
This is very implausible. The most commonly accepted theory is that jag is the same word as jag meaning a load – to have a load on is certainly a reasonable metaphor for drunkenness. As you can see in the link above, deoch does not sound like jag.