To shake a flannin was to fight. It is navvies' slang — the talk of the labourers who dug the canals and railways — and J. Redding Ware recorded it in Passing English of the Victorian Era (1909) with the briefest of definitions: To fight.

Ware does not explain the phrase directly. Instead he sends the reader to another entry, flannel-jacket, and that is where the story is.

The flannel jacket, he writes, was so standard among navvies that the garment became a name for the man: Familiar name for the gigantic navvy who, without exception, wears this garment. And the navvies did not say flannel. It was generally pronounced 'flannin', Ware notes, flannel being a hard word from Wales. He adds that the playwright Tom Taylor used the term in a scene of The Ticket-of-Leave Man.

So a flannin is a flannel jacket, and the jacket stands for the navvy who wore it. What shaking it meant exactly, Ware does not say. It may be the jacket shrugged off before a fight, or the man himself shaken by the collar — we have no source that settles it, so we leave both as guesses. Either way, once flannin was being shaken, the talking was over.

It is a reminder that a lot of slang is local. This was not the language of drawing rooms or even of London streets. It belonged to big gangs of working men on the cuttings and embankments, and Ware, collecting in 1909, was writing down a phrase from the century just ended.