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.
According to Daniel Cassidy’s work of false etymology, How The Irish Invented Slang, the word dick, an obsolete slang term for a detective, derives from the Irish word dearc, meaning an eye. This is how Cassidy explains it:
“The Pinkerton’s world-famous logo was the giant ‘All-Seeing Eye.’ The Pinkerton private ‘eye’ and labor union spy was christened a dick (dearc, an eye) by the Irish-speaking subjects of its gaze: Molly Maguires, Fenians, Knights of Labor, and Wobblies.”
The etymology of the word dick is not clear, though it is perhaps a corruption and shortening of ‘detective’. However, there are various ideas on the table and Grant Barrett goes through some of them in his treatment of Cassidy’s ‘research’: http://grantbarrett.com/humdinger-of-a-bad-irish-scholar
Wherever it does come from, the word dearc is not a reasonable suggestion for the origin of this word. Firstly, dearc is pronounced jarrick in northern dialects and something like darrick in southern Irish. It is not the usual word for eye in Modern Irish, which is súil. I doubt if most Irish speakers would even know the word.