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 claims that while jazz derives from teas, jazzy derives from the adjectival version teasaí. It seems obvious to me (and to any other sensible human being) that jazzy is simply an English adjectival form derived from jazz, just as a day of rain is a rainy day or a stew with lots of meat is described as meaty. Cassidy frequently did things like this. For example, look at his claims in relation to the supposed link between croak and Irish croch.