This ancient word was originally the Anglo-French pied-poudreux, meaning someone with dusty feet (in Scotland the term was sometimes translated into the delightfully Tolkienesque dustyfoot).
This was a graphic description of the state of someone travelling about on the unmade roads of medieval England. Piepowder was most often applied to itinerant merchants who toured the country to buy and sell at fairs.
Many of these fairs had been established by royal charters that gave rights to lords of the manor or religious houses to charge taxes and tolls and to enforce them. The enforcement was carried out in a rough-and-ready way through courts of piepowder (in Scotland they were sometimes known as courts of dusty feet).
These courts usually had jurisdiction over such matters as contracts, trespass and debts, and they sorted out trading disputes, punished theft and violence and did their best to keep order. Because few such fairs lasted more than three days, justice had to be swift and summary. The piepowder courts died out during the course of the nineteenth century along with the fairs that had brought them into being; the last is said to have been that in Hemel Hempstead in Hertfordshire, which last met in 1898.