The Collaborative International Dictionary of English v.0.48:

Testify \Tes"ti*fy\, v. i. [imp. & p. p. Testified; p. pr. &
   vb. n. Testifying.] [OF. testifier, L. testificari; testis
   a witness + -ficare (in comp.) to make. See -fy, and cf.
   Attest, Contest, Detest, Protest, Testament.]
   1. To make a solemn declaration, verbal or written, to
      establish some fact; to give testimony for the purpose of
      communicating to others a knowledge of something not known
      to them.
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            Jesus . . . needed not that any should testify of
            man, for he knew what was in man.     --John ii. 25.
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   2. (Law) To make a solemn declaration under oath or
      affirmation, for the purpose of establishing, or making
      proof of, some fact to a court; to give testimony in a
      cause depending before a tribunal.
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            One witness shall not testify against any person to
            cause him to die.                     --Num. xxxv.
                                                  30.
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   3. To declare a charge; to protest; to give information; to
      bear witness; -- with against.
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            O Israel, . . . I will testify against thee. --Ps.
                                                  l. 7.
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            I testified against them in the day wherein they
            sold victuals.                        --Neh. xiii.
                                                  15.
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The Collaborative International Dictionary of English v.0.48:

Testify \Tes"ti*fy\, adv.
   In a testy manner; fretfully; peevishly; with petulance.
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The Collaborative International Dictionary of English v.0.48:

Testify \Tes"ti*fy\, v. t.
   1. To bear witness to; to support the truth of by testimony;
      to affirm or declare solemny.
      [1913 Webster]

            We speak that we do know, and testify that we have
            seen; and ye receive not our witness. --John iii.
                                                  11.
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   2. (Law) To affirm or declare under oath or affirmation
      before a tribunal, in order to prove some fact.
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WordNet (r) 3.0 (2006):

testify
    v 1: give testimony in a court of law [syn: testify, attest,
         take the stand, bear witness]
    2: provide evidence for; "The blood test showed that he was the
       father"; "Her behavior testified to her incompetence" [syn:
       testify, bear witness, prove, evidence, show]

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "testify":
   acknowledge, affirm, allege, announce, argue, assert,
   assert under oath, asseverate, assure, attest, aver, avouch, avow,
   be indicative of, be significant of, be symptomatic of,
   bear witness, bespeak, betoken, book, bring forward,
   call to witness, certify, characterize, connote, declare,
   demonstrate, denominate, denote, depone, depose, differentiate,
   disclose, display, entail, express, give evidence, give token,
   guarantee, highlight, hint, identify, indicate, involve,
   kiss the book, manifest, mark, mean, mount, note, point to,
   proclaim, prove, reveal, say, show, signify, stand for, state,
   suggest, swear, swear by bell, swear in, swear the truth, swear to,
   swear to God, swear to goodness, symptomatize, symptomize,
   take oath, vouch, vouchsafe, vow, warrant, witness