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.
Cassidy explains this word as follows:
Can (slang), n., rear end; butt, the human rump.
Ceann (pron. k’an), n., (of person) end, extremity; rear end.
This is not true. Ceann means ‘head’. It can have the meaning of extremity in certain circumstances. Ó Dónaill gives a couple of uses of the word ceann where it can refer to the backside. For example Bhí a dhá cheann i dtalamh,(literally ‘his two ends were in the earth’) means he was bent low over his work, drudging. And the idiom Bhí ceann síos air (An end down was on him) means he was suffering from diarrhoea. However, these are specific idioms. It is quite wrong to suggest that ceann can be used in ordinary conversation to refer to the backside. No Irish speaker would say *Suigh ansin ar do cheann to mean Sit there on your bottom, and they would have to do this routinely for the word ceann to have been borrowed with the meaning of backside.
Back in the real world, Douglas Harper (https://www.etymonline.com/word/can) says that can has been a slang term meaning “toilet” since c. 1900, which is said to be a shortening of piss-can. He says that the meaning of “buttocks” is from c. 1910, perhaps extended from this. This may be right, or it may be wrong, but ceann does not mean ‘rear end’ in Irish so Cassidy’s claim is worthless.