There is a far land called the Wild Frontier, brutal and wretchedly poor.
In addition, the place is plagued by this group of bandits, in spite of the fact that the few posessions anyone has are more easily gained by hunting or farming.
One might ask what is the use of robbery, when nothing is worth taking? The answer is that the Dandy Highwaymen have not chosen to become bandits. Instead, it is a symptom of their madness.
In addition to its other misfortunes, the Wild Frontier has many outcrops of the magic metal adamantium. This substance, which flashes in the sun and grabs the attention of passers-by, drives those who touch it mad.
The victim is filled with a compulsion to become a bandit. In addition, on the few occasions when a Dandy Highwayman does get some small measure of wealth, their madness will drive them to spend it on clothes, from the few peddlers who travel through the Frontier.
There are two types of Dandy Highwayman. Whether they derive from two types of adamantium is unknown. The first, the 'Ant People', are almost all sterile in the manner of insects. The 'Sex People', by contrast, are able to reproduce normally. The two types appear identical to normal humans, but they are able to tell themselves apart, and a given group will be of one type.