Dic: undoing

• His lack of experience may prove to be his undoing.
• In the end, drink was his undoing.

One may compare the two different ways of approach to explain the expression:

1. N-SING : with poss | If something is someone's undoing, it is the cause of their failure. (= downfall)
2. be somebody's undoing: to cause someone's shame, failure etc.

.... Cobuild, LDOCE