Source : Webster's Revised Unabridged Dictionary (1913)

Undo \Un*do"\, v. t. [AS. und?n. See 1st {Un-}, and Do to
   perform.]
   1. To reverse, as what has been done; to annul; to bring to
      naught.

            What's done can not be undone.        --Shak.

            To-morrow, ere the setting sun, She 'd all undo that
            she had done.                         --Swift.

   2. 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. --Tennyson.

            Pray you, undo this button.           --Shak.

            She took the spindle, and undoing the thread
            gradually, measured it.               --Sir W.
                                                  Scott.

   3. 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.

            That quaffing and drinking will undo you, --Shak.

Source : WordNet®

undo
     v 1: cancel, annul, or reverse an action or its effect; "I wish I
          could undo my actions"
     2: deprive of certain characteristics [syn: {unmake}] [ant: {do}]
     3: cause the ruin or downfall of; "A single mistake undid the
        President and he had to resign"
     4: cause to become loose; "undo the shoelace"; "untie the
        knot"; "loosen the necktie" [syn: {untie}, {loosen}]
     5: remove the outer cover or wrapping of; "Let's unwrap the
        gifts!"; "undo the parcel" [syn: {unwrap}] [ant: {wrap}]
     [also: {undone}, {undid}]