verb (used with object), undid, undone, undoing.
to reverse the doing of; cause to be as if never done: Murder once done can never be undone.
to do away with; erase; efface: to undo the havoc done by the storm.
to bring to ruin or disaster; destroy: In the end his lies undid him.
to unfasten by releasing: to undo a gate; to undo a button.
to untie or loose (a knot, rope, etc.).
to open (a package, wrapping, etc.).
Archaic. to explain; interpret.

before 900; Middle English; Old English undōn; cognate with Dutch ontdoen. See un-2, do1

undoable, adjective

undo, undue.
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
undo (ʌnˈduː)
vb , -does, -doing, -did, -done
1.  (also intr) to untie, unwrap, or open or become untied, unwrapped, etc
2.  to reverse the effects of
3.  to cause the downfall of
4.  obsolete to explain or solve

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

O.E. undon "to unfasten and open" (a window or door), "to unfasten by releasing from a fixed position," from un- (2) + do. Undone "not accomplished" is recorded from c.1300; sense of "destroyed" is recorded from mid-14c.; the notion is of "to annul something
that was done." Undoing "action of bringing to ruin" is recorded from late 14c.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Now a small, dedicated group tries to undo some of the damage.
Needless to say, it has taken me a couple of days to undo that day.
They undo the cables, they're back in the cab, and away they go.
And to do it, they will have to undo all your efforts.
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