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 are various theories about the origins of gibberish. You can read some of them here: https://www.etymonline.com/word/gibberish
Cassidy claims that this comes from the ‘Irish’ geab ar ais, meaning ‘back talk, backward chat; fig. back-slanged speech.’ There is no evidence that the phrase geab ar ais has ever been used in the Irish language with any meaning, least of all ‘back-slanged speech’. You would think that ‘geab ar ais’ (if it existed) would be more likely to mean ‘backchat’ (i.e. impertinence) rather than backslang.