Source : Webster's Revised Unabridged Dictionary (1913)

Main \Main\, n. [AS. m[ae]gen strength, power, force; akin to
   OHG. magan, Icel. megin, and to E. may, v. ?. See {May}, v.]
   1. Strength; force; might; violent effort. [Obs., except in
      certain phrases.]

            There were in this battle of most might and main.
                                                  --R. of Gl.

            He 'gan advance, With huge force, and with
            importable main.                      --Spenser.

   2. The chief or principal part; the main or most important
      thing. [Obs., except in special uses.]

            Resolved to rest upon the title of Lancaster as the
            main, and to use the other two . . . but as
            supporters.                           --Bacon.

   3. Specifically:
      (a) The great sea, as distinguished from an arm, bay,
          etc.; the high sea; the ocean. ``Struggling in the
          main.'' --Dryden.
      (b) The continent, as distinguished from an island; the
          mainland. ``Invaded the main of Spain.'' --Bacon.
      (c) principal duct or pipe, as distinguished from lesser
          ones; esp. (Engin.), a principal pipe leading to or
          from a reservoir; as, a fire main.

   {Forcing main}, the delivery pipe of a pump.

   {For the main}, or {In the main}, for the most part; in the
      greatest part.

   {With might and main}, or {With all one's might and main},
      with all one's strength; with violent effort.

            With might and main they chased the murderous fox.
                                                  --Dryden.

Source : WordNet®

in the main
     adv 1: for the most part; "he is mainly interested in butterflies"
            [syn: {chiefly}, {principally}, {primarily}, {mainly}]
     2: without distinction of one from others; "he is interested in
        snakes in general" [syn: {generally}, {in general}] [ant:
        {specifically}]