(This is another piece which I have republished, edited and translated into Irish because of The Year of the Irish Language 2018. Seo píosa eile atá athfhoilsithe agam anseo, maraon le roinnt athruithe agus aistriúchán i nGaeilge, in ómós do Bhliain na Gaeilge 2018.)
Daniel Cassidy, in his crazy book How The Irish Invented Slang, claimed that the word goon, meaning an idiot (and later, a muscle-bound henchman) derives from the Irish word guan, meaning ‘a fool’. There are several problems with this. Firstly, Cassidy states that the English word is ‘origin unknown’, while most dictionaries (including the OED) regard it as a contraction of an earlier word goonie or gooney, which is known since the 16th century and means a fool or a large bird like an albatross. This seems perfectly reasonable and I can see no reason to prefer an Irish derivation to this English origin.
Secondly, guan is not a common word in Irish. It is not given at all in Ó Dónaill’s dictionary, and in Dinneen’s dictionary it is ascribed to Ó Neachtain’s manuscript dictionary of 1730. It is not found in the 7 million word Corpas na Gaeilge. The word guanach for silly or fanciful is certainly common and is given in all dictionaries but guan itself isn’t.
In other words, this is as stupid and unlikely as the rest of Cassidy’s nonsense.
Mhaígh Daniel Cassidy, ina leabhar craiceáilte How The Irish Invented Slang, go bhfuair an Béarla an focal goon, a raibh an chiall ‘amadán’ leis fadó (agus a fuair an chiall cúlaistín matánach ina dhiaidh sin), ón fhocal Gaeilge ‘guan’, a chiallaíonn amadán. Tá roinnt fadhbanna leis an mhíniú seo. Sa chéad áit, deir Cassidy go ndeir foclóirí an Bhéarla gur focal ‘origin unknown’ atá ann. Is bréag lom é sin. An chuid is mó de na foclóirí Béarla (an OED san áireamh), tá siad ar aon intinn gur giorrúchán é ar an tseanfhocal goonie nó gooney, atá ar taifead ón 16ú haois agus a chiallaíonn amadán nó éan mór cosúil leis an albatras. Tá an míniú seo iomlán réasúnta, dar liom féin, agus ní thuigim cén fáth a roghnódh duine ar bith an tsanasaíocht Ghaeilge in áit an chinn seo ón Bhéarla.
Ar an dara dul síos, ní focal coitianta guan sa Ghaeilge. Ní luaitear i bhfoclóir Uí Dhónaill é agus i bhfoclóir an Duinnínigh, deir sé go bhfuil sé i lámhscríbhinn fhoclóir Uí Neachtain a scríobhadh in 1730. Níl sé luaite i gCorpas na Gaeilge, corpas 7 milliún focal. Tá an focal guanach (amaideach) coitianta go leor, cinnte, ach níl an focal guan coitianta ar chor ar bith.
Lena rá i mbeagán focal, tá an ceann seo chomh bómánta neamhdhóchúil leis an chuid eile de raiméis Cassidy.
cassidyslangscam 
Straight off the top of my head, I’d guess that ‘gooney’ was the original word. Wherever it came from it seems to have had a respectable history in English. Then it either was or became understood as an adjective, hence ‘goon’ (noun) would be a back-formation, if not simply an abbreviation. The Irish ‘guanach’ would then be a loan with the native -ach adjective formative substituted for the equivalent Eng. -ey. Then ‘guan’ if it ever really existed would have been formed by dropping the ending to get the (imagined?) ‘original’ noun just as seems to have happened in English.
Seo mo sgeul, agus nam biodh breug ann, cha bu mhise a rinn e 😉
Tá mé cinnte go bhfuil an ceart agat, a chara. There is certainly no evidence for the existence of the word guan prior to 1730, from what I can see. The word guanach occurs in SG and in Irish with the same meaning but the 1730 dictionary is the only evidence in either language of guan existing as a noun. So where did it come from? As you say, the most likely explanation is that guan and guanach came from the English gooney, wherever that originates. It’s typical of Cassidy’s half-arsed etymological speculations that he didn’t even bother establishing which language the word first occurred in before announcing that it derived from Irish!
I hadn’t thought to look for ‘guanach’ in SG. There the core meaning seems to be ‘shaking, rocking, unsteady’ like a small stiff plant on a blustery day. So any resemblance to ‘gooney’ may just be accidental.
Oh, I’m going by McBain’s dictionary, which says silly or fanciful, which would be roughly the same as the Irish. Is that definition from Dwelly?
All the important SG dictionaries are brought together here :
http://www.faclair.com/
Don’t let the ‘beag’ put you off, it’s long since outgrown it’s name! Here’s the ‘about’ page, if you don’t see the English, scroll down.
http://www.faclair.com/GaelicDictionaryAbout.html#About
Maith thú! Rinne mé dearmad de na foinsí sin. Tá an chuid is mó acu ag aontú leis na foclóirí Gaeilge, de réir cosúlachta. 🙂