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 mystery about the origins of the word guffaw. It’s a Scottish term imitative of the sound of a hearty laugh, like ho-ho-ho in English or pá há (gáire) in Irish. See a brief account of its etymology here: https://www.etymonline.com/word/guffaw
According to the late Daniel Cassidy, this comes from gáire foghar, which he claims means: ‘a laughing sound or noise.’ In fact, Cassidy has got this the wrong way round. Gáire foghair (it needs to be in the genitive) would mean ‘a laugh of sound’, which doesn’t mean anything. It would have to be ‘a sound of laughter’, which would be foghar gáire. In other words, this ‘Irish’ phrase is completely fake and the genuine origin is well-known anyway.