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 Daniel Cassidy in his work of false etymology, How The Irish Invented Slang, the word jag, meaning a load for the back, comes from the Irish word tiach (as, according to him, does the word jack). There is no evidence in favour of this theory.
Jag originally meant a load of furze or whin, then later came to be a general term for a load: https://en.wiktionary.org/wiki/jag