Reversing or canceling something that was done, or the cause of someone's failure or ruin.
From Old English 'un-' (reversing an action) plus 'doing.' The prefix 'un-' is one of English's most productive prefixes, used to reverse almost any action, and 'doing' comes from 'do.'
The prefix 'un-' is so powerful in English that you can add it to nearly any verb to mean its opposite—'undo,' 'untie,' 'unlock'—making it one of language's most useful shortcuts!
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