In the play 'Macbeth' by William Shakespeare, the author shows us how Lady Macbeth wishes to be 'unsexed' in the sense that she wishes her femininity would not impede her resolve to have the king murdered. If she were a man, her blood would be 'thicker' (the Shakespeareans believed that certain 'humors' or emotions could be prevented from reaching the heart/brain by thicker blood.) Emotions which would have prevented Lady Macbeth from aiding, abetting or even committing the murders would have been pity, empathy, forgiveness and selflessness. An 'unsexed' woman would be stripped of all the gentler personality traits, she thinks, and better able to do evil deeds such as murdering a king through a lust for power.
Lady Macbeth asks them to do this (or wishes they could) because she wants to help her husband kill Duncan and do the other things that he needs to do in order to become king. Her request is based on the idea that women, by their nature, are unsuited for doing brutal things like that.
Lady Macbeth is worried that her husband lacks the guts and the drive to do what is necessary to take power. She thinks she has them, but she would need to stop being a woman in order to act on her impulses.
As a woman, she is too likely to feel guilt and remorse, she says. So she wants to be filled with cruelty like a man.
Come, you spirits
That tend on mortal thoughts, unsex me here
And fill me, from the crown to the toe, top-full
Of direst cruelty! Make thick my blood,
Stop up the access and passage to remorse,(45)
That no compunctious visitings of nature
Shake my fell purpose nor keep peace between
The effect and it!