Shakespeare uses androgyny in Macbeth to create confusion and reinforce the theme of strength and violence.
The word androgyny means having male and female traits. In Macbeth, there are two examples. First, the witches are genderless. Second, Lady Macbeth is often talked about as a man and asks to be a man.
When Macabeth and Banquo first see the witches, Banquo questions their gender. He also questions whether they really are inhabitants of the earth.
You should be women,
And yet your beards forbid me to interpret
That you are so. (Act 1, Scene 3, enotes pdf p. 12)
Some versions of the play have the witches played by men. They are women, but have male characteristics. This makes them strange, but also reinforces the idea of strength and power since men usually have the power in society.
Lady Macbeth, he says, will only have sons because she is so tough. She also seems to ask to become a man or have a man’s strength.
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! (Act 1, Scene 5, p. 20)
Lady Macbeth famously is compared to a man by Macbeth, and also seems to be asked to turn into a man or have men’s strength. Macbeth tells his wife this when she pushes him.
Bring forth men-children only,
For thy undaunted mettle should compose
Nothing but males. (Act 1, Scene 7, p. 24)
In each case, Lady Macbeth is seen as the driving force in the plan. She is the one who spurs her husband along and tries to convince him to kill Duncan. When he falters, she is the one who calls him a coward.
By making the women act less as women, Shakespeare is able to characterize them as he needs to. He is also making a statement about what women really are. Women can be strong, and can be violent.