The Collaborative International Dictionary of English v.0.48:

Undo \Un*do"\, v. t. [AS. und?n. See 1st Un-, and Do to
   perform.]
   1. To reverse, as what has been done; to annul; to bring to
      naught.
      [1913 Webster]

            What's done can not be undone.        --Shak.
      [1913 Webster]

            To-morrow, ere the setting sun,
            She 'd all undo that she had done.    --Swift.
      [1913 Webster]

   2. To loose; to open; to take to piece; to unfasten; to
      untie; hence, to unravel; to solve; as, to undo a knot; to
      undo a puzzling question; to undo a riddle. --Tennyson.
      [1913 Webster]

            Pray you, undo this button.           --Shak.
      [1913 Webster]

            She took the spindle, and undoing the thread
            gradually, measured it.               --Sir W.
                                                  Scott.
      [1913 Webster]

   3. To bring to poverty; to impoverish; to ruin, as in
      reputation, morals, hopes, or the like; as, many are
      undone by unavoidable losses, but more undo themselves by
      vices and dissipation, or by indolence.
      [1913 Webster]

            That quaffing and drinking will undo you, --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

undo
    v 1: cancel, annul, or reverse an action or its effect; "I wish
         I could undo my actions"
    2: deprive of certain characteristics [syn: unmake, undo]
       [ant: do, make]
    3: cause the ruin or downfall of; "A single mistake undid the
       President and he had to resign"
    4: cause to become loose; "undo the shoelace"; "untie the knot";
       "loosen the necktie" [syn: untie, undo, loosen]
    5: remove the outer cover or wrapping of; "Let's unwrap the
       gifts!"; "undo the parcel" [syn: unwrap, undo] [ant:
       wrap, wrap up]

Moby Thesaurus II by Grady Ward, 1.0:

237 Moby Thesaurus words for "undo":
   KO, abate, abolish, abrogate, annihilate, annul, answer, atomize,
   bare, beat, beat all hollow, beat hollow, best, bottom,
   break to pieces, bring down, bring to naught, bring to nothing,
   buffer, cancel, cancel out, cast off, clear, clear up, cleave,
   come to nothing, cook, counterbalance, crack, crush, dash, debauch,
   debug, decimate, decipher, decode, defeat, demolish, demoralize,
   deobstruct, destroy, detach, disassemble, disclose, disengage,
   disentangle, dish, disintegrate, dismantle, divine, do, do for,
   do in, doff, dope, dope out, douse, drop, drub, explain, expose,
   fathom, figure out, find out, find the answer, find the solution,
   fix, fragment, free, frustrate, get, get right, guess, guess right,
   have, have it, hide, hit it, hors de combat, interpret, invalidate,
   knock down, knock out, lambaste, lather, liberate, lick, loose,
   loosen, make mincemeat of, make out, manifest, negate, negativate,
   negative, neutralize, nullify, offset, ope, open, open the lock,
   outclass, outdo, outfight, outfox, outgeneral, outmaneuver,
   outpoint, outreach, outrun, outsail, outshine, outsmart, overcome,
   overreach, patefy, pick to pieces, plumb, prostrate, psych,
   psych out, pull in pieces, pull to pieces, pulverize, put, put off,
   puzzle out, quash, ravel, ravel out, raze, reduce to rubble,
   release, remove, rend, repeal, rescind, resolve, reveal, reverse,
   revoke, riddle, ruin, scuttle, settle, shake, shatter, shoot down,
   sink, skin, skin alive, slip out of, smash, solve, sort out, split,
   step out of, stultify, sunder, take apart, take off, tear apart,
   tear to pieces, tear to shreds, tear to tatters, thrash, throw off,
   thwart, torpedo, total, trim, triumph over, trounce, unbar, unbind,
   unblock, unbolt, unbrace, unbuckle, unbuild, unbutton, uncase,
   unchain, unclasp, unclench, unclog, unclutch, uncork, uncover,
   uncurtain, undrape, unfasten, unfetter, unfold, unfoul, unglue,
   unhook, unlace, unlatch, unleash, unlock, unloose, unloosen,
   unmake, unman, unnerve, unplug, unravel, unriddle, unroll,
   unscramble, unseal, unsheathe, unshut, unsnap, unstick, unstop,
   unstrap, unstring, untangle, untie, untwist, unveil, unweave,
   unwrap, unzip, upset, vitiate, void, weigh anchor, whip, work,
   work out, worst, wrack, wrack up, wreck