A benny, or ben, was a slang term for an overcoat in America. This is believed to derive from the word Benjamin, with the same meaning. Cassidy actually mentions the word Benjamin as an alternative to ben, but only at the end, which suggests that ben came first and Benjamin came later. This is not the case. Ben is a shortened form of Benjamin.
Apparently, in the eighteenth century, in England, the term for an overcoat was a Joseph, from the Biblical story of Joseph’s coat of many colours. By the early 19th century, the term Benjamin was also used in England of a smaller or tighter style of coat, because Benjamin was Joseph’s younger brother.
Cassidy does not mention these facts, opting instead to claim that benny or ben is from the Irish báinín, which Cassidy defines as:
Báinín (pron. bánín) n. a jacket or overcoat made of woolen cloth; any type of overcoat or jacket.
As usual, this is not an honest representation of the facts. A báinín is not primarily a coat or jacket. Báinín is a diminutive of the word bán, meaning white. The ‘little white’ of báinín was a type of rough, homespun woollen cloth. It can also be used to refer to a jacket or waistcoat made of báinín or bawneen cloth. It is not used of all types of jackets or coats. It is entirely unbelievable that people who had made it in the USA would refer to their finery as báinín/bawneen, cheap home-made cloth which was a badge of poverty. Here is the definition as given in Ó Dónaill’s dictionary:
báinín, m. (gs. ~, pl. ~í). 1. ~ (tíre), woven woollen cloth. ~ brocach, speckled homespun cloth. ~ bán, white homespun cloth. 2. Flannel. ~ dearg, glas, red, grey, flannel. 3. Jacket made of white homespun woollen cloth. F:Fear an bháinín (bháin), the Connemara labourer.