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.
Again, this is a minced oath that is used as a version of Jesus. Apparently Gee Whiz is attested from 1871. It should be noted that Jeez is a common variant of Gee.
Cassidy claims that this comes from Dia Uas, which, according to Cassidy, means Great God. This is a complete fabrication. The phrase Dia Uas does not exist in the Irish language. The word uas only exists as a prefix in Modern Irish, as you can see here: https://www.teanglann.ie/en/fgb/uas