Another ridiculous claim from Cassidy is that fink, which originally meant a company or government spy in labour organisations, is from the Irish fionnachtain. This is absurd. Fionnachtain means to find, discover or invent. There are specific terms for a spy such as néaladóir, spiaire agus brathadóir. Why wouldn’t Irish speakers have used one of them?
There is another problem with this word being the origin of fink, namely that it has three syllables, while fink only has one! And finnakhtun doesn’t really sound like fink, does it?
Then there is the problem that the real explanation is well-known. Fink almost certainly comes from the German word Fink, meaning a finch. Cassidy dismissed this explanation, as he did on many occasions with many words, but he didn’t tell his readers the full story and let them make up their own minds. The fact is that Fink was used in German student societies to mean a non-member of the group. In German there are also abusive compound words like Dreckfink, a shitbird. There were plenty of German-speaking people in NY in the 19th century and the word is one of those words that just sound appropriate. You would know that fink is not a good thing to be, even if you had never heard it before.
In short, this is another instance where Cassidy’s claim is just pure childish fantasy.