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.
The word cully is an old flash expression, which originally meant a man or friend in the 17th century but then came to mean a gull or sucker. There is no clear etymology for this word, though it has been linked to an earlier word cullion.
Daniel Cassidy in his book of false etymology How The Irish Invented Slang, says that the Irish cuallaí (earlier spelling cuallaidhe) is the origin of this word. The meaning is certainly close, as cuallaí means a companion or associate. In other words, there is nothing stupid about this claim. However, I think it is quite unlikely that there is a connection, as cuallaí is pronounced a little like the English coolie, not cully, and it would hard to explain why the sound of the word would be changed so much in the transition between the two languages.