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.
Cassidy says that this is ‘an oath or exclamation’. Although all reputable etymological sources regard this as a minced oath for ‘By God’ or ‘By God’s Body’ dating back to the 18th century, Cassidy doesn’t even mention this possibility. Instead, he links it to the genuine Irish phrase bíodh geall air, which means ‘I’ll bet’. This is not a great match in terms of sound (it is pronounced bee-oo gyal air) or meaning. Even the examples Cassidy gives work better if we assume that By Golly stands for By God than for I’ll bet you! in Irish.