Cassidy claimed that jazz comes from the Irish teas, meaning heat. He is not alone in claiming an Irish origin for the word jazz. Years ago, I remember someone saying to me that jazz comes from the Irish deas, meaning nice. I was sceptical of that claim and I’m just as sceptical of Cassidy’s. I suppose it is just possible but there is no evidence for it beyond a slight phonetic similarity. Cassidy also makes a basic mistake of pronunciation, in that he insists that teas is pronounced as jass, which it isn’t. Teas is pronounced chass, or tyass, but never jass. (In the book, he ganches on about something called the Rule of Tír, which I am fairly certain doesn’t exist and is not in any grammar book or textbook of Irish which I have seen).
There is no convincing solution to the problem of where the term jazz comes from. Some scholars insist that it was originally a sexual term which became applied to a type of music. They may be right, or they may be wrong. But there is no reason at all to associate it with an Irish word for heat (or nice).
I am also suspicious of the idea of the Irish ‘claiming’ jazz. I don’t dispute that individual Irish Americans had a big influence on the development of jazz but I wonder if Irish people were involved much in its inception. After all, the popular instruments among the Irish diaspora were the fiddle, the flute, the pipes and the whistle. The quintessential jazz instruments like brass and clarinet and drums and piano were really not part of the Irish music scene and most Irish music is in triple time, while most jazz is in quadruple time.
However, the most interesting thing about Cassidy and jazz for me is the debates on Wikipedia about the origin of the word. If you go to Wikipedia at this address: http://en.wikipedia.org/w/index.php?title=Jazz_(word)&offset=20081022201100&action=history you will find a number of debates involving a character who calls himself Medbh. I say himself, in spite of the fact that Medhbh(?)/Medb/Méabh is a woman’s name, because I am pretty certain that Medbh is really Daniel Cassidy. Read it yourself and make up your own mind but it ends up as a rant against the ‘dictionary dudes’ who have criticised Cassidy’s book. If Medbh’s comments were not written by Cassidy, then he or she had a stalker’s knowledge of Cassidy’s life and work and he or she used the same style of language. It is also a style of language found in other places by Cassidy himself under his own name, as well as by people who talked about Cassidy in the third person but were almost certainly sock-puppets for Cassidy (check out this exchange here http://www.wordorigins.org/index.php/forums/viewthread/663/ which is obviously from Cassidy himself and then compare it to Medbh’s hysterical rant below).
“You do not own the word “jazz” (teas) on Wikipedia or anywhere else. You are not balancing anything. Your article is replete with inaccuaracies and distortions. It is an embarassemnt. The attempt to marginalize Daniel Cassidy’s pioneering work on the word “jazz” and hundreds of other American vernacular words and phrases in his new book How the Irish Invented Slang: the Secret language of the Crossroads is pathetic. Cassidy’s book has been hailed by scores of respected academics, journalists, writers, and Irish language scholars, since its publication 3 months ago. See the Irish Times, The Irish Independent, The Belfast Telegraph, Irish News, The Derry Journal, RTE, Ireland’s national broadcaster, and Irish language publications like La Nua, Beo, and Foinse, as well as American media, including ABC radio, KPFA, WBAI, the SF Chronicle, and NY Observer,and this is just in the first weeks after publication. I shall continue to put up the Irish sanas of jazz. These last feeble attempts to censor Cassidy’s work are laughably pathetic. Let’s put it to mediation. I will provide 20 PUBLISHED articles supporting Cassidy’s thesis. All you have are the same old white boy cronies and Anglophile dictionary dudes.”
The strangest thing about this is the sheer ineptitude. After all, if you wanted to post a defence of your own book under a false identity, would you use highly distinctive phrases (sanas, dictionary dudes) which are associated with you in contexts where your name is given? Wouldn’t you try to adopt another persona, use a different voice to make your point? I would, certainly, but then I’m not barking mad …
cassidyslangscam 
Jazz was always considered the “negro music” of the US south and its northern diaspora following the great migration northwards.
Any attempts to make them or etymology “irish”, is just childish, desperate, needy, and not even worth addressing.
Daniel, I agree with you 100%. This book is nonsense from beginning to end and perhaps I shouldn’t have bothered addressing Cassidy’s claims in such detail – perhaps ignoring him would have been better. However, the success of this book is both puzzling and infuriating. It is currently at number 29 in the Amazon.com list of books on linguistics (which is a joke – Cassidy knew nothing about linguistics). That puts it ahead of John McWhorter’s excellent The Power of Babel and Steven Pinker’s The Stuff of Thought as well as a great many other books which are actually worth reading!
Yes, jazz is one of the many wonderful contributions made by African Americans to world culture and anyone who believes otherwise is an idiot.
I’m actually very glad that someone who actually speaks the Irish language addressed the book’s entries in detail. Please do not discontinue your work.
I feel Cassidy’s work was embarrassing, and an insult to the Irish population. This site provides an important counterbalance. Please continue to provide your detailed analyses.
Thanks for that, Daniel! I’m glad to know that people appreciate my efforts. I do intend to stop posting here soon but only because nearly every major entry in Cassidy’s book has been dealt with here or by other scholars and also because dealing with this level of folly on a regular basis is a bit depressing!