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, fears Lady Macbeth, he may not be up to the task of killing King Duncan.
Lady Macbeth imagines herself as a kind of vessel, and her eloquent malediction is her own vivid way of praying to be stripped of the feminine or perhaps of all her remaining humanity and filled completely, “from crown to toe,” with “direst cruelty” — i.e. the spirit of violence — so that she herself might help her hapless husband fulfill the dirty deed.
It is interesting to note also here 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 “all the perfumes of Arabia cannot sweeten Lady Macbeth’s little hand.”