Green’s Dictionary of Slang

1. a card sharp or dice fraud.

c.1698		B.E. Dict. Canting Crew n.p.: Nickum, a Sharper.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].

2. a cheating landlord (although the aspersion can extend to any dishonest retailer).

c.1698		B.E. Dict. Canting Crew n.p.: Nickum, [...] a Rooking Ale-house or Innkeeper, Vintner, or any Retailer.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].

3. a thief.

1892		Aberdeen Eve. Exp. 27 July 3/1: [The] witness explained that he was a ‘nickum’ [...] Francis found a ‘quid’ of tobacco on him, and accused him of stealing a penny to buy it.
1910		Dover Exp. 6 May 8/2: ‘Master Nickum’ (thief).