The Collaborative International Dictionary of English v.0.48:

Main \Main\ (m[=a]n), a. [From Main strength, possibly
   influenced by OF. maine, magne, great, L. magnus. Cf.
   Magnate.]
   1. Very or extremely strong. [Obs.]
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            That current with main fury ran.      --Daniel.
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   2. Vast; huge. [Obs.] "The main abyss." --Milton.
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   3. Unqualified; absolute; entire; sheer. [Obs.] "It's a man
      untruth." --Sir W. Scott.
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   4. Principal; chief; first in size, rank, importance, etc.;
      as, the main reason to go; the main proponent.
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            Our main interest is to be happy as we can.
                                                  --Tillotson.
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   5. Important; necessary. [Obs.]
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            That which thou aright
            Believest so main to our success, I bring. --Milton.
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   By main force, by mere force or sheer force; by violent
      effort; as, to subdue insurrection by main force.
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            That Maine which by main force Warwick did win.
                                                  --Shak.
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   By main strength, by sheer strength; as, to lift a heavy
      weight by main strength.

   Main beam (Steam Engine), working beam.

   Main boom (Naut.), the boom which extends the foot of the
      mainsail in a fore and aft vessel.

   Main brace.
      (a) (Mech.) The brace which resists the chief strain. Cf.
          Counter brace.
      (b) (Naut.) The brace attached to the main yard.

   Main center (Steam Engine), a shaft upon which a working
      beam or side lever swings.

   Main chance. See under Chance.

   Main couple (Arch.), the principal truss in a roof.

   Main deck (Naut.), the deck next below the spar deck; the
      principal deck.

   Main keel (Naut.), the principal or true keel of a vessel,
      as distinguished from the false keel.
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   Syn: Principal; chief; leading; cardinal; capital.
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