What's done cannot be undone

To remember this quote use the following mnemonic.

What have you done, that cannot be ever undone.

"What's done cannot be undone." Act 5, scene 1, line 59 just before Lady Macbeth exits.

This is spoken by Lady Macbeth during her famous sleepwalking scene. It reflects a moment of profound realization and despair. 

At this point in the play, Lady Macbeth is overwhelmed by guilt and madness over the crimes she and Macbeth have committed, particularly the murder of King Duncan.

The phrase means that once an action has been taken it cannot be reversed or erased. In this context, it refers to the irreversible nature of their deeds - the bloodshed, betrayal, and violence they have set in motion. 

Earlier in the play, Lady Macbeth was the driving force behind Macbeth's ambition, urging him to seize power without hesitation. However, by this point, she is consumed by remorse and the psychological weight of their actions. The line underscores the inevitability of consequences and the futility of trying to escape the moral and emotional fallout of their choices. It also serves as a broader commentary on the inescapable nature of guilt and the permanence of one's actions.