Not tied up, or the past tense of the verb 'to untie,' meaning to unfasten something that was knotted or bound.
From Old English 'tīed,' past participle of 'tēon' (to tie), with the prefix 'un-' added to reverse the action. This is a straightforward Germanic word formation.
The prefix 'un-' appears in thousands of English words and always means 'reverse' or 'opposite'—it's like a linguistic undo button that lets us express the opposite of almost any action.
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