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.
Geister is a word that Cassidy found in one of Jonathon Harrington Green’s books on gambling, published in the mid 19th century. Green defined a geister as an extra thief. In other words, it seems likely to be linked to the German word geist, meaning a ghost. There are a number of expressions in English where someone is marginal to the work that use the term ghost: a ghost writer is an invisible writer who writes the book for the ostensible author; to ghost someone at work means to follow them to learn the job.
Cassidy changed its meaning to ‘a thief’s confederate’, which gives a completely different impression. According to Cassidy, it comes from the Irish gastaire: which means (again, according to Cassidy) ‘a tricky person; a cunning, impudent fellow.
In fact, Dinneen’s dictionary claims that it means ‘a tricky person’.
Ó Dónaill’s dictionary says that this means ‘a smart, impudent fellow.’
There is no evidence for a connection between Irish gastaire and English geister. In fact, the sound match is not great (geister, gastarra) and the meanings even less so. In any case, why would a cheeky or tricky individual be a good match for the meaning of ‘an extra thief brought in to do a job?’