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.
Doodle is an obsolete slang term for an idiot. You can find an account of its use and etymology here:
Cassidy claims that this derives from the Irish word, dúdálaí, defined by Ó Dónaill as:
dúdálaí, m. (gs. ~, pl. -aithe). Shy, self-conscious, person; stupid person.
This is given in Dinneen’s Dictionary (first published in 1904) as dúdálaidhe (which would be pronounced the same as modern dúdálaí) but there is no evidence that it existed before the late 19th century or that it is not derived from English doodle. In other words, it is far more likely to be derived from the English doodle than to be the source of that word.