Real Dictionary

untie
verb: If you untie something such as string or rope, you undo it so that there is no knot or so that it is no longer tying something.
verb: If you untie something that is tied to another thing or if you untie two things that are tied together, you remove the string or rope that holds them or that has been tied round them.

untie in American English
to loosen, undo, or unfasten (something tied or knotted)
intransitive verb: to become untied
transitive verb: to loose or unfasten (anything tied); let or set loose by undoing a knot

untie in British English
verb: to unfasten or free (a knot or something that is tied) or (of a knot or something that is tied) to become unfastened