Cassidy claims that this derives from the Irish gáire foghar. Gáire foghar is not a real phrase. As usual, there is no evidence that anyone had ever used it before Daniel Cassidy came along with his pocket dictionary and his head full of fantasies. For someone to convincingly claim that a word comes from a phrase in another language, they should at least be able to prove that the phrase exists in the source language. If it did exist, it would mean something like ‘a laugh of voices’ or ‘a laugh of sounds’, which is decidedly odd. There are plenty of less confusing ways of saying ‘a loud laugh’. And then there is the inconvenient little truth that De Bhaldraithe’s dictionary gives three words as the equivalent of guffaw in Irish – scolgháire, glamgháire and scairteadh gáire. None of which sound anything like gáire foghar.
So where does guffaw come from? It’s probably an imitation of the sound of someone bursting out laughing, like I did the first time I read Cassidy’s book, before I realised how many people take this rubbish seriously. Since then, the guffaw has just turned into a low growl of annoyance.