chief or principal a main road our main objective by main force using sheer strength [Middle English, from attributive use of Old English mægen strength, reinforced by Old Norse megn strong]
a principal pipe, duct, or cable carrying gas, electricity, or water(the mains) the central source of water, gas, or electricity supplied to an area for use by a large number of consumersa mainmasta mainsail(the main) archaic a mainland They … tried to make a landing on the main — Byron(the main) literary or archaic the open sea to traverse climes beyond the Western Main — Goldsmitharchaic physical strengthin/for the main on the wholewith all one's might and main with all one's strength
in the game of hazard1 (3), a number exceeding four and not exceeding nine called by a player before throwing the dice [prob from main2]