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 the late Daniel Cassidy, in his etymological hoax, How The Irish Invented Slang, the English word square in the sense of honest is not from the English word square meaning right-angled and even. It is really from the Irish ’s cóir, which according to Cassidy is the opposite of scam, the Irish ’s cam. While cóir does mean right or just, the idea that a (partial) phrase like ’s cóir would be borrowed between languages is nonsense. Borrowings between languages follow certain patterns. Borrowings are nearly always individual words and the vast majority of them are nouns.
Square is also an ancient word in English. The noun (set-square, a tool for carpenters) dates back to the 13th century and derives from Old French esquire “a square, squareness,” from Vulgar Latin.
As an adjective, it had acquired the meaning of honest or fair by the 1560s.