This is a famous and often misunderstood passage from Macbeth (Act 1, Scene 5), spoken by the unforgettable Lady Macbeth, who says:

The raven himself is hoarse
That croaks the fatal entrance of Duncan
Under my battlements. Come, you spirits
That tend on mortal thoughts, unsex me here,
And fill me from the crown to the toe topful
Of direst cruelty.

She’s referring, of course, not to sex or the sex act but to the fact that her husband is becoming more and more squeamish about the business of murder — as a result of which he may not, Lady Macbeth fears, be up to the task of killing King Duncan after all, as the two had originally planned.

Lady Macbeth imagines herself here as a kind of vessel, and her eloquent malediction is her own vivid way of praying to be bled of all her nurturing, feminine virtues — and perhaps of all her remaining humanity. She asks that in its place she be filled completely, “from crown to toe topful,” with “direst cruelty” — i.e. the spirit of violence — so that she herself might help her hapless husband fulfill the dirty deed.

It’s interesting to note that the prefix un– appears with abnormal regularity throughout the entirety of Macbeth, almost as if the characters are continually trying to undo the horrible deeds they’ve done — “to cancel reality by appending negatives,” as the Shakespearean Michael Macrone felicitously phrased it.

Of course, once the deeds are accomplished they can never be unaccomplished — as, to her unadulterated horror, Lady Macbeth sees with undeniably clarity, moments after her prayer is answered, her wish granted, and, unwitting no more, she stands unmasked before her image in the mirror: an accomplice to murder and a co-conspirator to regicide, frantically scrubbing her homicidal hand in hopes of unstaining her skin now unexpungably stained with blood — “Out, damned spot! Out!” — but it’s too late: too late to unsay the words she’s already said, to unanswer the prayer that’s already been answered, to unenact the deeds her unsexed self has helped enact — her unhelpful husband, unsexed and unmanned in his own way, a way of unhealthy mind and unsound spirit, condemned by his own conscience for his unconscionable deeds which has culminated in “murther most foul and unnatural” — so that Lady Macbeth, unmoored, alone and sinking into “multitudinous seas incarnadine,” unraveling to such an extent that she feels death may undo her at any moment, realizes in unequivocal terror and then unhappy resignation that she scrubs her hand unnecessarily: the unspeakable act she’s helped commit can never be uncommitted, and “all the perfumes of Arabia cannot sweeten this little hand.”