The Collaborative International Dictionary of English v.0.48:

Untie \Un*tie"\, v. t. [AS. unt[=y]gan. See 1st Un-, and
   Tie, v. t.]
   [1913 Webster]
   1. To loosen, as something interlaced or knotted; to
      disengage the parts of; as, to untie a knot.
      [1913 Webster]

            Sacharissa's captive fain
            Would untie his iron chain.           --Waller.
      [1913 Webster]

            Her snakes untied, sulphurous waters drink. --Pope.
      [1913 Webster]

   2. To free from fastening or from restraint; to let loose; to
      unbind.
      [1913 Webster]

            Though you untie the winds, and let them fight
            Against the churches.                 --Shak.
      [1913 Webster]

            All the evils of an untied tongue we put upon the
            accounts of drunkenness.              --Jer. Taylor.
      [1913 Webster]

   3. To resolve; to unfold; to clear.
      [1913 Webster]

            They quicken sloth, perplexities untie. --Denham.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Untie \Un*tie"\, v. i.
   To become untied or loosed.
   [1913 Webster]

WordNet (r) 3.0 (2006):

untie
    v 1: undo the ties of; "They untied the prisoner" [syn: untie,
         unbrace, unlace] [ant: bind, tie]
    2: cause to become loose; "undo the shoelace"; "untie the knot";
       "loosen the necktie" [syn: untie, undo, loosen]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "untie":
   cast off, detach, disembarrass, disembroil, disencumber, disengage,
   disentangle, doff, free, liberate, loose, loosen, release, remove,
   take off, unbar, unbind, unbolt, unbridle, unbuckle, unbutton,
   uncage, unchain, unclasp, undo, unfasten, unfetter, ungag, unglue,
   unhandcuff, unhobble, unlace, unlash, unlatch, unleash, unlock,
   unloose, unloosen, unmanacle, unmuzzle, unpen, unpinion,
   unscramble, unshackle, unstick, unstrap, untangle, untether,
   untruss, untwine, unyoke, weigh anchor