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.
This is another strange and deeply unlikely etymology proposed by Daniel Cassidy in his book How The Irish Invented Slang. This is clearly another minced oath (a disguised curse or blasphemy). In this case, it is almost certainly a corruption of Holy Mary. This has become Holy Molly (a common form of Mary in Ireland) and then it has been altered to rhyme with holy, so that it became Holy Moly.
Cassidy denies this and claims that it comes from a mixed English and Irish phrase, Holy Moladh, where moladh means praise. There is no evidence of anyone ever using Moladh Naofa! or Moladh! or any other phrase with moladh in the Irish language as an exclamation, so there is absolutely no evidence for Cassidy’s claim here. The word moladh is pronounced molloo or molla, depending on dialect, so it is no great match for Moly in terms of sound either.