Sharpers who drop a piece of gold, which they pick up in the presence of some unexperienced person, for whom the trap is laid, this they pretend to have found, and, as he saw them pick it up, they invite him to a public house to partake of it: when there, two or three of their comrades drop in, as if by accident, and propose cards, or some other game, when they seldom fail of stripping their prey.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Going Upon the Dub * Gold FinderNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes