Reversed or unfastened something that was done or tied; past tense of 'undo.'
From Old English 'un-' (meaning reversal) combined with 'do' or 'did.' This is a simple Germanic construction that's been in English since its earliest forms.
The prefix 'un-' is one of English's oldest and most productive word-builders—you can 'un-' almost anything and be understood. This flexibility is why English speakers can handle constant change; we just add 'un-' and keep going without needing official permission.
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