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.
It is difficult to understand why this word is even in Cassidy’s book. There has never been any doubt about its origin. It is from the Irish bean sí, fairy woman. There has never been any doubt about its Irish origin and this is acknowledged in all English dictionaries.
Incidentally, about 60 years ago there was a comprehensive overhaul of the Irish system of orthography. At that time, the earlier spelling, bean sidhe was superseded by bean sí. While not everybody has been enthusiastic about the spelling reform, it has met with general acceptance and is the form favoured by the education system and government, and this is the form that should be given in discussions of words of Irish etymology in dictionaries.