The Collaborative International Dictionary of English v.0.48:

Proof \Proof\, a.
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   1. Used in proving or testing; as, a proof load, or proof
      charge.
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   2. Firm or successful in resisting; as, proof against harm;
      waterproof; bombproof.
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            I . . . have found thee
            Proof against all temptation.         --Milton.
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            This was a good, stout proof article of faith.
                                                  --Burke.
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   3. Being of a certain standard as to strength; -- said of
      alcoholic liquors.
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   Proof charge (Firearms), a charge of powder and ball,
      greater than the service charge, fired in an arm, as a gun
      or cannon, to test its strength.

   Proof impression. See under Impression.

   Proof load (Engin.), the greatest load than can be applied
      to a piece, as a beam, column, etc., without straining the
      piece beyond the elastic limit.

   Proof sheet. See Proof, n., 5.

   Proof spirit (Chem.), a strong distilled liquor, or mixture
      of alcohol and water, containing not less than a standard
      amount of alcohol. In the United States "proof spirit is
      defined by law to be that mixture of alcohol and water
      which contains one half of its volume of alcohol, the
      alcohol when at a temperature of 60[deg] Fahrenheit being
      of specific gravity 0.7939 referred to water at its
      maximum density as unity. Proof spirit has at 60[deg]
      Fahrenheit a specific gravity of 0.93353, 100 parts by
      volume of the same consisting of 50 parts of absolute
      alcohol and 53.71 parts of water," the apparent excess of
      water being due to contraction of the liquids on mixture.
      In England proof spirit is defined by Act 58, George III.,
      to be such as shall at a temperature of 51[deg] Fahrenheit
      weigh exactly the 12/13 part of an equal measure of
      distilled water. This contains 49.3 per cent by weight, or
      57.09 by volume, of alcohol. Stronger spirits, as those of
      about 60, 70, and 80 per cent of alcohol, are sometimes
      called second, third, and fourth proof spirits
      respectively.

   Proof staff, a straight-edge used by millers to test the
      flatness of a stone.

   Proof stick (Sugar Manuf.), a rod in the side of a vacuum
      pan, for testing the consistency of the sirup.

   Proof text, a passage of Scripture used to prove a
      doctrine.
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