1 loosen, loose, open, unfasten, unhook, unlace, unzip, unsnap, unbutton, untie, unpin; unlock, unbolt:
Help me undo my blouse. Don't undo the gate - you'll let the dogs out.
2 unwrap, uncover, open, untie, unbind:
Why have you allowed the children to undo their Christmas presents a week early?
3 cancel, annul, rescind, nullify, void, declare null and void, reverse, invalidate:
If one could undo things there would be no such thing as regret.