This will be a short post, because Cassidy’s claim is so obviously wrong for so many reasons. Cassidy claims that nag (meaning an old or clapped-out horse) comes from the Irish n-each, meaning a horse. He states that each is the Irish for horse and that n-each is a ‘form of’ this. English is a relatively uninflected language. Words tend to be hard pebbles of meaning which change little, so it is hard for English speakers to understand that particles like n- have no meaning outside of a particular context and so they are very unlikely to be borrowed. It is a little like taking the phrase ‘They shouldn’t ‘ve gone’ and deciding to take out the ‘ve gone bit and treat it as a meaningful unit. N-each is not a word. And in any case, each is not the word for horse in modern Irish; this is capall or beithíoch depending on the dialect. And while the origin of nag is not clear, there is no doubt that it has been in English for a very long time. It was first recorded as nagge (pronounced naga) in the 14th century, so even if n-each existed and even if it were a good candidate for the origin of nag, it would be unlikely to be the source of the word because there is no convincing way that it could have entered English that early. In short, this is typical Cassidy horse feathers and anyone who tries to tell you otherwise is flogging a dead horse.