To let fall, with design, a piece of gold or silver, in order to draw in and cheat the person who sees it picked up; the piece so dropped is called a dropt cog.
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.
Drop * Drop In the EyeNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes