Green’s Dictionary of Slang

(UK Und.) ‘silver’ plate that is in fact mainly copper with a thin silver coating.

1665		Head Eng. Rogue I 186: This here is a George plateroon, being all Copper within, and only a thin Plate about it.
1714		A. Smith Lives of Most Noted Highway-men, etc. I 205: What the professors of this hellish Art [i.e. counterfeiting] call George Plateroon, is all Copper within, with only a thin Plate [i.e. of silver] about it.