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.
The word Gee used as an exclamation is certainly what is known as a ‘minced oath’. In other words, it is a way of avoiding a taboo word by saying something that slightly resembles the offensive term. I was under the misapprehension that this is short for God, because G is the first letter, but the overwhelming body of opinion has it that this is short for Jesus: https://www.etymonline.com/word/gee
Cassidy thinks this comes from the Irish Dia, which is pronounced with a j sound in the North, as jeea. There is no evidence for this and Gee is exactly like the first syllable of Jesus. Furthermore, Irish speakers tend to say a Dhia (the vocative – oh God!) which is pronounced a yeea, rather than ‘Dia!’