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.
Gism or gasm are slang words for energy or for semen. The origin of these expressions is unknown.
Daniel Cassidy claims that they come from the ‘Irish’ teas ioma, which he claims means ‘an abundance of heat, passion, excitement.’ Cassidy thinks the word iomaí (or ioma) is an ordinary adjective which can follow a noun. It isn’t and it can’t: https://www.teanglann.ie/en/fgb/iomaí
In other words, this is not just a non-existent phrase in Irish. This is an impossible phrase. It is a piece of made-up nonsense which means nothing.