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.
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            Sacharissa's captive fain
            Would untie his iron chain.           --Waller.
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            Her snakes untied, sulphurous waters drink. --Pope.
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   2. To free from fastening or from restraint; to let loose; to
      unbind.
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            Though you untie the winds, and let them fight
            Against the churches.                 --Shak.
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            All the evils of an untied tongue we put upon the
            accounts of drunkenness.              --Jer. Taylor.
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   3. To resolve; to unfold; to clear.
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            They quicken sloth, perplexities untie. --Denham.
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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]