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.
This is another clear example of Cassidy’s dodgy methodology, the way that he massaged and distorted and edited the facts to make his case look even slightly convincing.
He quotes the OED as saying “Of obscure origin; possibly … Gael [sic] and Irish gob, beak, mouth …” (OED.) No page number or indication of what edition of the OED he’s talking about.
Having tried to demonstrate that the OED is deficient, he then says that American Slang lexicographers Wentworth and Flexner, in the Dictionary of American Slang, ‘put gob back into the gob of the Irish’.
The facts about gob are well-known and well-established. The fact is, there are two gobs in English (at least). They come from different origins. In other words, the origins of gob are more complex than Cassidy claimed. Not very complex. Anyone of normal ability would have been able to understand them but the late Daniel Cassidy didn’t do complexity.
One gob is the gob of ‘a gobbet of food’. It means a mouthful rather than a mouth. This dates back to the 13th century in English, apparently coming from the French verb gober meaning to gulp or swallow down. This came into French from Celtic (probably Gaulish) and is related to the Irish word gob. The other is a borrowing from either Irish or Scottish Gaelic gob, meaning beak or mouth. This is quite ancient in English too. It is found as far back as the 1540s.
Now, Wentworth and Flexner are probably right in a sense. Gob is used a lot in Hiberno-English, and when people in Irish areas of New York or Philadelphia said ‘I’ll give you a dig in the gob’, there is every chance that this came directly from Irish or from Hiberno-English. However, the Gaelic and Celtic roots of this word were never in any doubt and the Oxford English Dictionary did not doubt this either, whatever Cassidy claimed.
cassidyslangscam 
I had a message from Donnacha Delong, surprisingly:
I don’t know about the OED, I don’t have one to hand, but dictionary.com – derived from the Random House dictionary – gives this origin of gobshite: “The first records of the term gobshite come from around 1946. It combines the word gob, a British slang word for “mouth,” and the word shite, a British variation of the word shit, a vulgar word meaning “feces” or “excrement.” https://www.dictionary.com/browse/gobshite No mention of the Irish gob (and related “clab” – as in dun do chlab – shut your mouth).
Actually, he’s wrong about gob being related to clab, but not wrong about the use of British being inappropriate here. It’s very much an Irish expression. According to the Oxford Ulster Dictionary, gobshite is used to refer to a bird, the skua. This is presumably what Dinneen (1927) referred to as seabhac cac faoileann, the seagull shite hawk, so called (in Mayo) because it pursues seagulls until they release their excrement which the hawk eats. This is not true, of course, but it is a long-standing piece of folklore. In fact, they harass other birds to steal food from them. The Latin name of the genus is stertocorius (of dung) because of this story.
In other words, not only is this the origin of gobshite, it’s probably also the origin of gobhawk and shitehawk.
I have had a reply from Donnacha DeLong:
How am I “wrong” about clab – which is closer in sound to gob in English – being a possible point of origin than gob – pronounced gub in Irish (like bog and clog).
My point was about the origin of gob, not necessarily of gobshite, when I reference clab. Clab, in its most common usage in my youth – “dun do chlab” is linguistically very close to gob depending on the canúint.
It is, of course, very possible that the origin of gobshite is different from the origin of gob, being a later adoption from Irish/Hiberno-English.
There is so little research on this topic that it’s impossible to be certain about any of it.
Uh … you’re wrong because … you’re not right. Your message is a bit garbled but I’ll do my best to answer it. Leaving aside gobshite, you seem to be claiming that the words gob and clab in Irish are related. That’s the meaning I take from your message.
Gob and clab are similar in meaning but no more alike in sound than God and clad, so I can’t really understand why you would even think they are related.
The Irish word gob means a bird’s beak, or any beak-like object (the nib of a pen, the prow of a boat) and is also used as a contemptuous term for the mouth. According to Wiktionary, it comes from Old Irish gop, which comes in turn from Proto-Celtic *gobbos.
Clab is found in both Irish and Scottish Gaelic with the meaning of a large mouth. It is found in Electronic Dictionary of the Irish language with both the spelling clab and the spelling clap. McBain’s Scottish Gaelic Dictionary suggests that it comes from the English word clap (clappe in Middle English), meaning a clap, loud noise, the tongue of a bell. Clappe was also found in Middle English in the phrase ‘holden clappe’, to hold your tongue, which is a good match for dún do chlab. I am not 100% sure that this is the origin of the Irish and Gaelic word clab but it seems likely and there don’t seem to be any other candidates.
That being the case, if gob (including the first element of gobshite) comes through Old Irish from Proto-Celtic gobbos while clab probably comes from Middle English clappe, then they are probably not related and looking at the form of the two words, there is no good reason to think that they are related.
As for the claim that there is so little research that it’s impossible to be sure, as I think I’ve demonstrated above, the information is there if you go looking for it. The idea that ‘there isn’t enough information so we get to believe whatever we want’ is the kind of thing I expect to hear from Anti-Vax nutters or Qanon followers or supporters of Graham Hancock, not from a journalist.