The Collaborative International Dictionary of English v.0.48:

Confirm \Con*firm"\, v. t. [imp. & p. p. Confrmed; p. pr. &
   vb. n. Confirming.] [OE. confermen, confirmen, OF.
   confermer, F. confirmer, fr. L. confirmare; con- + firmare to
   make firm, fr. firmus firm. See Firm.]
   1. To make firm or firmer; to add strength to; to establish;
      as, health is confirmed by exercise.
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            Confirm the crown to me and to mine heirs. --Shak.
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            And confirmed the same unto Jacob for a law. --Ps.
                                                  cv. 10.
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   2. To strengthen in judgment or purpose.
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            Confirmed, then, I resolve
            Adam shall share with me in bliss or woe. --Milton.
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   3. To give new assurance of the truth of; to render certain;
      to verify; to corroborate; as, to confirm a rumor.
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            Your eyes shall witness and confirm my tale. --Pope.
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            These likelihoods confirm her flight. --Shak.
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   4. To render valid by formal assent; to complete by a
      necessary sanction; to ratify; as, to confirm the
      appoinment of an official; the Senate confirms a treaty.
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            That treaty so prejudicial ought to have been
            remitted rather than confimed.        --Swift.
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   5. (Eccl.) To administer the rite of confirmation to. See
      Confirmation, 3.
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            Those which are thus confirmed are thereby supposed
            to be fit for admission to the sacrament. --Hammond.

   Syn: To strengthen; corroborate; substantiate; establish;
        fix; ratify; settle; verify; assure.
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WordNet (r) 3.0 (2006):

confirm
    v 1: establish or strengthen as with new evidence or facts; "his
         story confirmed my doubts"; "The evidence supports the
         defendant" [syn: confirm, corroborate, sustain,
         substantiate, support, affirm] [ant: contradict,
         negate]
    2: strengthen or make more firm; "The witnesses confirmed the
       victim's account" [syn: confirm, reassert]
    3: make more firm; "Confirm thy soul in self-control!"
    4: support a person for a position; "The Senate confirmed the
       President's candidate for Secretary of Defense"
    5: administer the rite of confirmation to; "the children were
       confirmed in their mother's faith"

Moby Thesaurus II by Grady Ward, 1.0:

204 Moby Thesaurus words for "confirm":
   OK, accede, accept, acclimate, acclimatize, accommodate, accredit,
   accustom, acquiesce, adapt, addict, adjust,
   administer the Eucharist, affirm, amen, anoint, approve, assay,
   assent, assure, attest, audit, authenticate, authorize, autograph,
   back, back up, be sponsor for, bear out, bed, beef up, bind,
   bolster, bond, brace, brace up, break, break in, bring to test,
   buttress, case harden, certify, check, check and doublecheck,
   check out, check over, chrism, circumstantiate, clinch, collate,
   condition, consent, corroborate, cosign, countersecure,
   countersign, cross-check, cut and try, deep-dye, define, do duty,
   document, domesticate, domesticize, double-check, embed, encourage,
   endorse, engraft, engrave, ensure, entrench, essay, establish,
   etch, experiment, familiarize, fix, fortify, found, gentle, gird,
   give a try, give a tryout, give permission, give the go-ahead,
   give the imprimatur, give thumbs up, ground, guarantee, guaranty,
   habituate, harden, have a go, housebreak, impact, implant, impose,
   impress, imprint, infix, ingrain, initial, inscribe, insure, inure,
   invigorate, jam, justify, lay hands on, lodge, make good, minister,
   naturalize, nerve, notarize, officiate, orient, orientate, pack,
   painstakingly match, pass, pass on, pass upon, perform a rite,
   perform service, permit, plant, play around with, practice upon,
   print, probate, prop, prove, put to trial, ratify, recheck,
   recognize, refresh, reinforce, reinvigorate, research,
   restrengthen, road-test, root, rubber stamp, run a sample, sample,
   sanction, say amen to, scrutinize comparatively, seal, season,
   seat, second, secure, set, set in, settle, shake down, shore up,
   sign, sign and seal, sign for, sponsor, stamp, stand behind,
   stand up for, steel, stereotype, stiffen, strengthen, subscribe,
   subscribe to, substantiate, support, sustain, swear and affirm,
   swear to, take to, tame, taste, temper, test, toughen, train,
   triple-check, try, try it on, try out, undergird, underpin,
   undersign, underwrite, uphold, validate, verify, visa, vise, vouch,
   warrant, wedge, witness, wont