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.
Dinneen’s Irish dictionary gives various meanings for snab, including a snap, an end or fragment, a spell or turn (cf. “a cold snap”). The book also gives snap with the meaning “a snapping, a sudden assault or seizure.”
Daniel Cassidy, in his etymological hoax, How The Irish Invented Slang, claims that the word snap is originally an Irish word which was borrowed into English. While snap in the sense of a snapping, a sudden assault or seizure, a snap, an end or fragment is found in Irish, the evidence is very clearly in favour of this being an English word borrowed into Irish for a number of reasons.
There is no evidence for the existence of the word snap in Irish before the modern era. (You can check this on eDIL.) Furthermore, snap is recorded in the sense of a snap or sudden bite in English from the late 15th century, and probably derives from Dutch or Low German snappen. It is related to Germanic words like snout. Snap in the sense of a sudden change of the weather is a natural extension of snap in the sense of a sudden bite.