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.
It is well-known and admitted by all dictionaries that slogan is of Gaelic origin, though most sources rightly trace it to Scottish Gaelic. The phrase sluagh-ghairm or slua-ghairm (NOT sluagh ghairm – it is a compound word and therefore needs to be written with a hyphen) means call to a host and was the traditional practice of summoning warriors to form an army in the Scottish highlands. There is nothing controversial about this etymology.
Cassidy is wrong in claiming that this is an Irish expression rather than Scottish Gaelic, though the term makes sense in both languages. However, to the best of my knowledge, there is no evidence of the use of slua-ghairm in Irish. There is a word slógadh, which also derives from a variant of slua, which means roughly the same thing – it is used now as the equivalent of the English mobilisation.
Some sources claim that the word slogan derives from an Irish term sluaghán. This is given as a historical term (Hist.) by the lexicographer de Bhaldraithe, whose dictionary was published in 1959. However, there does not seem to be any source for this word. I cannot find any reference to it in Irish before de Bhaldraithe and I would presume that de Bhaldraithe probably realised that there was a link between slogan and slua(gh), so he invented sluaghán as a probable source, not realising that it really derives from Scottish Gaelic slua-ghairm through various manglings in English such as sloghorne. If anyone has any information that contradicts this, I would be interested to hear it.
Since de Bhaldraithe first used it, its spelling has been modernised to sluán in Irish, and it is found in modern dictionaries like focloir.ie as a translation for slogan alongside words like mana and rosc catha. If my theory about de Bhaldraithe is correct, this is what linguists call a ghost word, a word deriving from a mistake or misunderstanding, like the word cigire in Irish or gravy in English. (Look it up if you don’t believe me!)