One of the clearest signs of Cassidy’s poor methodology is his failure to establish any clear parameters for his research. The title of the book suggests that it is about American slang and the way that as Irish speakers poured into the cities of North America during the 18th and 19th centuries, their Irish language might have given rise to slang terms in English. And many people insist that this is what Cassidy did and what his theories were all about.
But if this is what the book is about, how can you include words which were plainly a part of English centuries before any European (apart from the odd Viking) had set foot in North America? This is what Cassidy did, over and over again. Sneeze, mayhem, bicker, booze. All of these words go back a long, long way and have clear etymologies which Cassidy sometimes cited and then ignored or sometimes just ignored. Looking at it logically, which Cassidy was obviously incapable of doing, words which were mainstream in English by the 17th century have nothing to do with Cassidy’s supposed thesis, because they were learned by Irish speakers just as they learned the rest of the English language, regardless of their origin. There are also increasing problems with claiming an Irish origin the further back you go in time because there is no evidence of significant Irish immigration to England before the 18th century, so how would these words have transferred from the Irish-speaking community to English? Cassidy scoffs at the frequent derivations from Dutch or Flemish claimed by lexicographers but these are actually very easy to justify. There was a lot of immigration from Holland and Flanders in the east of England in the Middle Ages. This is historical fact.
One of Cassidy’s stupidest claims is that giggle comes from gíog gheal, which (if it existed) would mean something like a bright squeak. There is no evidence of anyone ever using the phrase gíog gheal in Irish, either to mean a laugh or a giggle or anything of the type. The phrase doesn’t exist, any more than people in English routinely talk about brightsqueaking when they have a giggle.
Furthermore, we have already said that where words have German cognates they are obviously long-standing and well-established words in English. German has a word gickeln which means almost exactly the same as giggle and sounds very like it. Giggle, gickeln. Against a fake phrase invented by Cassidy and completely unattested and which couldn’t be used in the same way as gickeln and giggle, as both noun and verb. As usual, it’s nonsense. It can be shown to be nonsense. All you have to do is access a German or Dutch dictionary on line and find the cognate and Cassidy’s fake derivation gets blown out of the water.