The Collaborative International Dictionary of English v.0.48:

Avouch \A*vouch"\, v. t. [imp. & p. p. Avouched (?); p. pr. &
   vb. n. Avouching.] [OF. avochier, LL. advocare to recognize
   the existence of a thing, to advocate, fr. L. advocare to
   call to; ad + vocare to call. Cf. Avow to declare,
   Advocate, and see Vouch, v. t.]
   1. To appeal to; to cite or claim as authority. [Obs.]
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            They avouch many successions of authorities. --Coke.
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   2. To maintain a just or true; to vouch for.
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            We might be disposed to question its authenticity,
            it if were not avouched by the full evidence.
                                                  --Milman.
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   3. To declare or assert positively and as matter of fact; to
      affirm openly.
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            If this which he avouches does appear. --Shak.
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            Such antiquities could have been avouched for the
            Irish.                                --Spenser.
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   4. To acknowledge deliberately; to admit; to confess; to
      sanction.
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            Thou hast avouched the Lord this day to be thy God.
                                                  --Deut. xxvi.
                                                  17.
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The Collaborative International Dictionary of English v.0.48:

Avouch \A*vouch"\, n.
   Evidence; declaration. [Obs.]
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         The sensible and true avouch
         Of mine own eyes.                        --Shak.
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WordNet (r) 3.0 (2006):

avouch
    v 1: admit openly and bluntly; make no bones about [syn: avow,
         avouch] [ant: disavow]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "avouch":
   acknowledge, admit, affirm, allege, announce, annunciate, argue,
   assert, assever, asseverate, assure, attest, aver, avow,
   bear witness, certify, confess, confirm, contend, corroborate,
   countersign, declare, depone, depose, disclose, enunciate, express,
   express the belief, give evidence, guarantee, have, hold, insist,
   issue a manifesto, lay down, maintain, make a promise, manifesto,
   own, pledge, plight, predicate, proclaim, profess, promise,
   pronounce, protest, put, put it, say, set down, speak, speak out,
   speak up, stand for, stand on, state, submit, swear, testify,
   troth, underwrite, vouch, vow, warrant, witness