A person's undoing is their downfall or ruin. In classic stories, a character's greatest strength sometimes becomes their undoing: the clever fox gets caught because he gets too confident in his cleverness, or the proud warrior loses because he refuses to ask for help. When something is someone's undoing, it means that thing caused their failure or defeat. A student's failure to read the instructions before a test might be their undoing for that test. A football player's injury might never heal properly and become the undoing of their once-promising career.