Real Dictionary

undo
If a person, organization, or plan is undone by something, that thing causes their failure.
If you undo something that is closed, tied, or held together, or if you undo the thing holding it, you loosen or remove the thing holding it.
To undo something that has been done means to reverse its effect.

undo in American English
to release or untie (a fastening)
transitive verb: to reverse the doing of; cause to be as if never done

undo in British English
verb: to untie, unwrap, or open or become untied, unwrapped, etc