The Collaborative International Dictionary of English v.0.48:

Falsify \Fal"si*fy\, v. i.
   To tell lies; to violate the truth.
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         It is absolutely and universally unlawful to lie and
         falsify.
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         South.
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The Collaborative International Dictionary of English v.0.48:

Falsify \Fal"si*fy\, v. t. [imp. & p. p. Falsified; p. pr. &
   vb. n. Falsifying.] [L. falsus false + -ly: cf. F.
   falsifier. See False, a.]
   1. To make false; to represent falsely.
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            The Irish bards use to forge and falsify everything
            as they list, to please or displease any man.
                                                  --Spenser.
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   2. To counterfeit; to forge; as, to falsify coin.
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   3. To prove to be false, or untrustworthy; to confute; to
      disprove; to nullify; to make to appear false.
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            By how much better than my word I am,
            By so much shall I falsify men's hope. --Shak.
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            Jews and Pagans united all their endeavors, under
            Julian the apostate, to baffle and falsify the
            prediction.                           --Addison.
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   4. To violate; to break by falsehood; as, to falsify one's
      faith or word. --Sir P. Sidney.
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   5. To baffle or escape; as, to falsify a blow. --Butler.
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   6. (Law) To avoid or defeat; to prove false, as a judgment.
      --Blackstone.
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   7. (Equity) To show, in accounting, (an inem of charge
      inserted in an account) to be wrong. --Story. Daniell.
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   8. To make false by multilation or addition; to tamper with;
      as, to falsify a record or document.
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WordNet (r) 3.0 (2006):

falsify
    v 1: make false by mutilation or addition; as of a message or
         story [syn: falsify, distort, garble, warp]
    2: tamper, with the purpose of deception; "Fudge the figures";
       "cook the books"; "falsify the data" [syn: fudge,
       manipulate, fake, falsify, cook, wangle,
       misrepresent]
    3: prove false; "Falsify a claim"
    4: falsify knowingly; "She falsified the records" [ant:
       correct, rectify, right]
    5: insert words into texts, often falsifying it thereby [syn:
       interpolate, alter, falsify]