A shabby, mean fellow; a term said to be derived from a number of dirty, tattered roguish boys, who attended at the Horse Guards, and Parade in St. James's Park, to black the boots and shoes of the soldiers, or to do any other dirty offices. These, from their constant attendance about the time of guard mounting, were nick-named the black-guards.
BLACK GUARD
Meaning of BLACK GUARD in English
Slang English vocab. Английский сленговый словарь. 2012