The Collaborative International Dictionary of English v.0.48:

Main \Main\, n. [AS. m[ae]gen strength, power, force; akin to
   OHG. magan, Icel. megin, and to E. may, v. [root]103. See
   May, v.]
   1. Strength; force; might; violent effort. [Obs., except in
      certain phrases.]
      [1913 Webster]

            There were in this battle of most might and main.
                                                  --R. of Gl.
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            He 'gan advance,
            With huge force, and with importable main.
                                                  --Spenser.
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   2. The chief or principal part; the main or most important
      thing. [Obs., except in special uses.]
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            Resolved to rest upon the title of Lancaster as the
            main, and to use the other two . . . but as
            supporters.                           --Bacon.
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   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.
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   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.
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            With might and main they chased the murderous fox.
                                                  --Dryden.
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WordNet (r) 3.0 (2006):

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