Guinea droppers, cheats, sharpers. To sweeten to decoy, or draw in. To be sweet upon; to coax, wheedle, court, or allure. He seemed sweet upon that wench; he seemed to court that girl.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Sweet Heart * SwellNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes