Apart from its use for a bread roll, the word bun has a number of uses in English slang. One use, which dates back about a hundred years, is in phrases like ‘to have a bun on’, which means to be mildly intoxicated. Another, more common use, is buns meaning buttocks. This is more recent and it is always used in the plural. Daniel Cassidy, in his moronic book How The Irish Invented Slang, claims that both these terms come from the Irish word bun, meaning base or foundation.
Is there any chance that Cassidy is right about this? Firstly, bun is not used in modern Irish for buttocks. However, there is some evidence that the Scottish Gaelic word bun was borrowed hundreds of years ago into the Scots language as a word for a rabbit’s or hare’s tail or scut (this is also believed to be the origin of bunny for a rabbit) and that by extension bun is used in Scots dialects (in the singular) for a bottom because of this. This is also found in Ulster dialects, along with derivatives like bundie, a childish term for the bottom. However, it seems to me obvious that the more modern version buns as in “nice buns” is not likely to be related to this. Buns look like buttocks and this is the likely origin of the term.
As for “having a bun on”, the origin of this phrase is unknown. Cassidy lifts the meaning foundation out of context and claims that this would mean a basic level of drunkenness. This is one hell of a stretch and of course, bun is not used in Irish in this way. There are plenty of terms for levels of drunkenness in Irish, and a mild drunkenness would be described with terms like ar bogmheisce and meidhreach.
As usual, this is just another foolish distortion of the facts. Cassidy was just a sad, deluded crank with no talent and no qualifications.