undoing

What it means

1. The act of loosening or unfastening
2. Ruin; defeat, (also) that which causes defeat or ruin.

   In the end, his jealousy was his undoing.

   2 more examples

   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.

   1912 October, Edgar Rice Burroughs, “Tarzan of the Apes”, in The All-Story, New York, N.Y.: Frank A. Munsey Co., →OCLC; republished as chapter 5, in Tarzan of the Apes, New York, N.Y.: A[lbert] L[evi] Burt Company, June 1914, →OCLC

   The extra 30 minutes could not separate the sides and led to a nerve-shredding finale that has so often been England's undoing, with a dismal record of just one win in seven shootouts at major tournaments before this.

   2018 July 3, Phil McNulty, “Colombia 1 - 1 England”, in BBC Sport

3. Annulment; reversal

In a sentence

In the end, his jealousy was his undoing.

Where it comes from

1. Middle Englishundoinge
2. Englishundoing

From Middle English undoinge, undoynge, ondoynge; equivalent to undo + -ing.