Those horse-boys and unmilitary folk, such as cooks with their pots, pans, and other kitchen utensils, which travel with an army, and greatly impede its march.
Gifford, in his edition of Ben Jonson, says: “In all great houses there were a number of dirty dependents, whose office it was to attend the wool-yards, sculleries, etc. Of these the most forlorn were selected to carry coals to the kitchen. They rode with the pots and pans, and were in derision called the black-guards.”
In the Lord Steward's office a proclamation (May 7th, 1683) begins thus: “Whereas ... a sort of vicious, idle, and masterless boyes and rogues, commonly called the Black-guard, with divers other lewd and loose fellows ... do usually haunt and follow the court. ... Wee do hereby strictly charge ... all those so called, ... with all other loose, idle ... men ... who have intruded themselves into his Majesty's court and stables ... to depart upon pain of imprisonment.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894