Princeton's WordNet

1. undo(verb)

   cancel, annul, or reverse an action or its effect

   "I wish I could undo my actions"

2. unmake, undo(verb)

   deprive of certain characteristics

3. undo(verb)

   cause the ruin or downfall of

   "A single mistake undid the President and he had to resign"

4. untie, undo, loosen(verb)

   cause to become loose

   "undo the shoelace"; "untie the knot"; "loosen the necktie"

5. unwrap, undo(verb)

   remove the outer cover or wrapping of

   "Let's unwrap the gifts!"; "undo the parcel"

Kernerman English Learner's Dictionary

1. undo(verb)ʌnˈdu

   to unfasten sth

   Undo your coat.

2. undoʌnˈdu

   to reverse the effects of sth

   It took a long time to undo all his mistakes.

Wiktionary

1. undo(Verb)

   To reverse the effects of an action.

   Fortunately, we can undo most of the damage to the system by the war.

2. undo(Verb)

   To unfasten.

   Could you undo my buckle for me?

Webster Dictionary

1. Undo(verb)

   to reverse, as what has been done; to annul; to bring to naught

2. Undo(verb)

   to loose; to open; to take to piece; to unfasten; to untie; hence, to unravel; to solve; as, to undo a knot; to undo a puzzling question; to undo a riddle

3. Undo(verb)

   to bring to poverty; to impoverish; to ruin, as in reputation, morals, hopes, or the like; as, many are undone by unavoidable losses, but more undo themselves by vices and dissipation, or by indolence

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