This is a famous and often misunderstood line 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, and so he may not, fears Lady Macbeth, be up to the task of killing King Duncan after all, as the two of them 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 stripped of all the nurturing feminine virtues she contains — 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 in 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 — though of course once the deeds are done, like all deeds, whether good or bad, they can never be undone, as to her horror Lady Macbeth herself realizes, shortly after her prayer is answered, wish granted, and King Duncan lies dead at her feet — dead by Lady Macbeth’s hand. Once more with inimitable eloquence and her precise, poetic way with words, Lady Macbeth accurately observes that her homicidal hand is forever now stained with blood — nothing can wash it away — even as she scrubs her hand and frets and scrubs it over and over, and even as she grows more and more frantic in her failed attempts to wash away the blood, so that at last in resignation notes aloud that “all the perfumes of Arabia cannot sweeten this little hand.”