The Collaborative International Dictionary of English v.0.48:

Undoing \Un*do"ing\, n.
   1. The reversal of what has been done.
      [1913 Webster]

   2. Ruin. "The utter undoing of some." --Hooker.
      [1913 Webster]

WordNet (r) 3.0 (2006):

undoing
    n 1: an act that makes a previous act of no effect (as if not
         done)
    2: loosening the ties that fasten something; "the tying of bow
       ties is an art; the untying is easy" [syn: untying,
       undoing, unfastening]

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "undoing":
   Waterloo, abasement, affliction, annulment, bane, beating, blight,
   bloodbath, blue ruin, breakup, cancellation, carnage, collapse,
   conquering, conquest, consumption, counterbalancing, crash, curse,
   damnation, deathblow, debacle, debasement, decimation, defeat,
   degradation, depredation, deprivation, descent, desolation,
   despoilment, despoliation, destroyer, destruction, devastation,
   disassembly, disgrace, disintegration, dismantlement,
   dismemberment, disorganization, disruption, dissolution, downfall,
   drubbing, failure, fall, frustration, havoc, hecatomb, hiding,
   holocaust, humiliation, invalidation, lambasting, lathering,
   licking, mastery, misfortune, mortification, neutralization,
   nullification, offsetting, overcoming, overthrow, overturn,
   perdition, quietus, ravage, ruin, ruination, shambles, shame,
   slaughter, smash, spoliation, stripping, subdual, subduing,
   subjugation, thrashing, thwarting, trimming, trouble, trouncing,
   vandalism, vanquishment, vitiation, voiding, waste, whipping,
   wrack, wrack and ruin, wreck