Green’s Dictionary of Slang

a pickpocket’s accomplice who jostles the victim while the pickpocket actually performs the theft; a shoplifter’s accomplice; thus as v. to jostle or impede a target.

1717		see stand gammon below.
1718		C. Hitchin Regulator 20: A Bulk or Gammon, alias that is he that jostles up to a Man, whilst another picks his Pocket.
1757		Proc. Old Bailey 20 Apr. 148: Yates. He said to his wife, the person coming along here is a fine lady or a fine woman, she said Gammon. Q. What did she mean by that? Yates. That was for us to get before the lady, it is a cant term, made use of by pick pockets, and it meant that she should not go too fast while she took her watch.
1768		(con. 1710–25) Tyburn Chronicle II in Groom (1999) xxviii: A Bulk or Gammon He that jostles a Man, while another picks his Pocket.
1777		J. Fielding Thieving Detected 19: There is generally two goes together, one is the Knuckle, and the other the Gammon, (the person who Stalls for him). [Ibid.] 50: One of them, which they call the Gammon, takes off the attention of the shop-keeper.
1786		Whole Art of Thieving [as cit. 1768].