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English definition of “undo”

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undo

verb [T] uk   /ʌnˈduː/ ( present participle undoing, past tense undid, past participle undone) us  

undo verb [T] (UNFASTEN)

B2 to unfasten something that is fastened or tied: Can someone help me to undo my seat belt?
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undo verb [T] (REMOVE EFFECTS)

B2 to remove the good or bad effects of an action or several actions: I did a really tough aerobics class and then went out for a meal and undid (all) the good work! It's very difficult to undo the damage that's caused by inadequate parenting in a child's early years.
(Definition of undo from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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