Green’s Dictionary of Slang

1. (UK gambling) in the game of hazard, a number on a given face of a dice: 5, 6, 7, 8, or 9.

1832

Satirist (London) 22 Jan. 31/1: There on the dice what is called five mains; that is — 5, 6, 7, 8, 9.

2. (Aus.) in dice gambling, the combined score of the two thrown dice.

1919

Mirror (Sydney) 31 Aug. 8/2: The manipulator, by sleight of hand, inserts in the pot the set he wants. If he wants a high main, in go the highs, and the lows if a small count is desired.

3. see mainline n. (2)

4. see main man n. (3)