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.
Daniel Cassidy claimed that the English word brisk derives from the Irish briosc, the primary meaning of which is brittle. It does have some other meanings, such as brisk and lively, but these are probably meanings borrowed from English. The usual claim about the English word brisk is that it derives from the French word brusque, as in this item from Etymonline:
This seems the most likely explanation. There is no evidence for an Irish origin.