ETYM as. undoen. Related to Un-, and Do to perform.
(Irregular preterit, past participle: undid, undone).
1. To cancel or reverse an action or its effect.
2. To make undone; revert to the original condition or situation.

To reverse the last action—for example, to undo a deletion, thus restoring deleted text to a document. Many application programs enable the user both to undo and to redo an action. See also undelete (definition 1).