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.
Cant was the criminal argot, the language of the beggars and thieves in Merry England, though sometimes it was also used to mean hypocritical or wheedling speech of any kind. You can find a good discussion of its origins here: https://www.etymonline.com/word/cant
Most dictionaries say that the word comes from the Latin cantare, to sing, though some trace it to a pair of puritanical preaching brothers called Cant. It has also been suggested (many times) that cant comes from the Irish or Scots Gaelic cain(n)t, meaning speech. This suggestion is not new and was not invented by Cassidy. For example, it was given already in R. McCutcheon’s Modern Language Notes in 1921.
It is not impossible that this claim is correct but as we have stated, it makes no difference because the claim had been made long before Cassidy was born.