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.
Glim is an old cant or flash expression for an eye or a candle or lamp.
Daniel Cassidy claims that it comes from either gealaim, which means I light, I illuminate, or else from geal-laom, a fake compound word invented by Cassidy himself, composed of geal meaning bright and laom, which means blaze or flash. In the case of gealaim, the -aim part is not an intrinsic part of the word. Other versions of this word include gealadh and gealaíonn, and neither of them is likely to be anything to do with glim. Geal-laom is completely unrecorded and Cassidy provides no examples or sources for his claim that it ever existed.
Back in the real world, glim is almost certainly either a version of gleam or a contraction of glimmer.