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, she 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 as well. 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 in Macbeth, almost as if the characters are continually trying to undo the horrible deeds they’ve done — “to cancel reality by appending negatives,” the Shakespearean Michael Macrone felicitously phrased it. Of course, once the deeds are done they can never be undone — as, to her horror, Lady Macbeth understands, shortly after her prayer is answered and her wish granted, and she now stands unmasked before her mirrored image: a murderer and regicide, frantically fretting and scrubbing her homicidal hands in hopes of washing away the blood that now stains them — “Out, damned spot! Out!” — but it’s too late now: too late to unanswer the prayer that’s been answered, to undo the deeds her unsexed self has done — her unmanned husband unwilling, unable to undertake them — so that Lady Macbeth, drowning alone in “multitudinous seas incarnadine now “ll the perfumes of Arabia cannot sweeten this little hand.”