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.
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. In English, 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.
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 (neither of which is the case), 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.