Green’s Dictionary of Slang

a counterfeit silver coin, e.g. a shilling.

1665		Head Eng. Rogue I 186: It is called a black Dog, with Queen Elizabeth’s Head thereon, which is only Pewter double washed.
1706		Luttrell in Ashton Reign Queen Anne II 225: The art of making black dogs, which are shillings, or other pieces of money, made only of Pewter, double wash’d [F&H].
1714		A. Smith Lives of Most Noted Highway-men, etc. I 205: Black Dogs, which are Shillings or other Pieces of Money made only of Pewter, double washed.
1730		Swift Drapier’s Letters III 44: Such as were coined in small numbers [...] as Butcher’s half-pence, black-dogs, and the Like .