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.
There is no real evidence for the origin of this word. It crops up first in America in the 1830s, with a number of different meanings. You can find an account of these facts here, on Douglas Harper’s excellent blog: https://www.etymonline.com/word/dingbat
Daniel Cassidy, in his work of false etymology, How The Irish Invented Slang, claims that it derives from the Irish duine bocht, a poor person. A hobo or tramp is one of the meanings of dingbat, but doesn’t seem to be the first or the main meaning. The pronunciation is reasonably close but not perfect. Listen to the sound files for duine and bocht here.
In other words, this is just possible but there is no evidence at all in its favour.
Incidentally, in earlier versions of his theories, Cassidy claimed that dingbat came from duine bod, which he thought meant a tramp. It fact, if it existed, it would mean something like ‘person of penises’.