Green’s Dictionary of Slang

a confidence trick based on deliberately dropping a guinea and swindling the dupe who picks it up.

1699		N. Ward London Spy V 112: The knack of Sweetning; or the most Ingenious Dexterity of Picking Pockets.
1714		A. Smith Lives of Most Noted Highway-men, etc. I 165: One Humphrey Jackson, a Butcher, who [...] went upon the sweetening Lay of Luck in a Bag.
1714		T. Lucas Lives of the Gamesters (1930) 179: The Sweetning Lay, which is decoying young gentlemen to be bit, or cheated by old sharpers.
1755		The Tricks of the Town Laid Open (4 edn) 61: Guinea-Dropping, or Sweetning, is a paltry little Cheat that was introduced to the World about sixty Years ago.
1839		(con. 1724) W.H. Ainsworth Jack Sheppard (1917) 223: A man has a mind to educate a hopeful child in [...] the knack of sweetening.