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.
Mow is a Scots and Northern English verb meaning to copulate with: https://www.dsl.ac.uk/entry/snd/mow_v1_n1
It goes back as far as the 16th century and has no connection with the obscure Irish word moth (pronounced moh), which Cassidy claims as the origin of the word. Moth is an ancient literary term, which is only used in modern Irish in compound words like mothchat, which means a male cat (a tomcat). Moth anciently meant a penis but this is not found in modern Irish at all and in any case, this doesn’t mean it could be used as a verb for sexual intercourse. In fact, there is a verb mothú in Irish, which is common and means to feel, to hear, to perceive. The word mothú does not provoke sniggers in Irish speakers because the word moth for penis does not exist in modern Irish.
It seems likely that mow is really linked to the verb meaning to cut grass, as terms for copulation in several languages use the metaphor of flattening, words such as laying in English and clárú in Irish.