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Webster 1913 Edition

Noun.

The reversal of what has been done.

Ruin.

“The utter undoing of some.”

Hooker.

ppr.

Reversing what has been done; ruining.

, n.

The reversal of what has been done.

Ruin; destruction.

undoing ‎(plural undoings)

That which defeats.
His fatal flaw was his undoing. In a sense he defeated himself.
- 1912: Edgar Rice Burroughs, Tarzan of the Apes, Chapter 5
  So far as the ape was concerned, Sabor reasoned correctly. The little fellow crouched trembling just an instant, but that instant was quite long enough to prove his undoing.

That which defeats

undoing