There is a joke at the heart of every sausage, and the Victorians gave it a name.

Bags o' mystery are sausages. J. Redding Ware, who collected the slang of the era in Passing English of the Victorian Era (1909), explains why with a straight face: they are called that because no man but the maker knows what is in them.

The bag is the casing — Ware is precise about it: the gut which contained the chopped meat. The mystery is everything else.

He also gives the joke a date and a reason. The phrase, he says, took its rise about 1850, long before the present system of market-inspection was organised — that is, in a time when what went into a sausage really was between the butcher and his conscience. Then he adds the most interesting detail: the name remained long after sausages were fairly wholesome. The suspicion outlived the problem, as suspicions do.

It was not a rare phrase, either. Thirty-five years before Ware, John Camden Hotten's Slang Dictionary (1874) listed bags of mystery as another phrase in frequent use, meaning sausages and saveloys.

Ware's own example sentence is a little window into the street: a complaint that sellers were being run in for putting rotten gee-gee — horse — into the bags of mystery. It is not a nice thought. It is also exactly why the name stuck.