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]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "falsify":
   alter, be untruthful, belie, burlesque, camouflage, caricature,
   change, cheat, color, contort, contradict, contravene, cook,
   deceive, deny, disguise, distort, doctor, draw the longbow,
   dress up, embellish, embroider, equivocate, evade, exaggerate,
   fake, fib, fudge, garble, gild, gloss, gloss over, lie, lie flatly,
   mask, miscite, miscolor, mislead, misquote, misreport,
   misrepresent, misstate, misteach, overdraw, overstate, palter,
   parody, pervert, prevaricate, shift, shift about, slant,
   speak falsely, story, strain, stretch the truth, tell a lie,
   titivate, traverse, travesty, trick out, trump up, twist,
   understate, varnish, warp, whitewash, wrench