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.
As a verb, the English word means to fail to keep up. As a noun, the word lag means a delay. This is a very old word in English, dating back to the early 16th century. You can find some more information about its etymology here:
As you can see, there is some doubt about its origins but it seems to be related to a Scandinavian root. For example, there is a Norwegian verb lagga, meaning to move slowly, and/or to a dialect English term for the last one in a race, who was referred to as the lag or laggie.
From Cassidy’s treatment of this word, it is clear that he has nothing original to say on the subject. The OED had apparently already discussed the supposed origin of the word from a Celtic source such as Welsh llag (or llac) or Gaelic lag, and dismissed it as highly improbable. Cassidy, of course, from a position of complete ignorance of linguistics and the Celtic languages, dismisses the OED’s concerns and states that “the Irish lag (weak, feeble; a weak person or thing) and its verb lagaigh (pron. lagí, to weaken, slacken) first lag into English slang and cant in the 16th century”.
The Irish for “my horse started well but soon lagged” is “thosaigh mo chapall go maith ach níorbh fhada gur thit sé chun deiridh”. Nothing to do with lag or lagaigh. (And lag is not a noun meaning a weak person in Irish, it’s an adjective).