The act of loosening or unfastening
Ruin; defeat, that which causes defeat or ruin.
His fatal flaw was his undoing. In a sense he defeated himself.
Annulment; reversal
To reverse the effects of an action.
Fortunately, we can undo most of the damage to the system by the war.
To unfasten.
Could you undo my buckle for me?
To impoverish or ruin, as in reputation; to cause the downfall of.