Meaning of “undo” in the English Dictionary

"undo" in British English

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undoverb [ T ]

uk /ʌnˈduː/ us /ʌnˈduː/ present participle undoing, past tense undid, past participle undone

(Definition of “undo” from the Cambridge Advanced Learner’s Dictionary & Thesaurus © Cambridge University Press)

"undo" in American English

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undoverb [ T ]

us /ʌnˈdu/ present participle undoing, past tense undid /ʌnˈdɪd/ , past participle undone /ʌnˈdʌn/

undo verb [ T ] (UNFASTEN)

to unfasten something that is closed or tied:

He undid the top button of his shirt and loosened his tie.

undo verb [ T ] (REMOVE EFFECTS)

to remove the effects of something that happened earlier:

It’s difficult to undo the damage caused by a father who abandons his child.

(Definition of “undo” from the Cambridge Academic Content Dictionary © Cambridge University Press)