Macbeth believes that Lady Macbeth's "undaunted mettle"—that is, her courageous strength —is more appropriate for a man than for a woman. Therefore, if a child is to be composed of Macbeth's personality (that of a powerful war hero) and Lady Macbeth's, the child should be male. Macbeth may be concerned that a daughter born of the two would be unhealthy in the overwhelming strength of personality that she would have; obviously, expectations for men and women were different in Jacobean England than they are today.

Interestingly, however, this line is usually played as a compliment in stage productions. The implication is that Lady Macbeth's strength makes her so worthy that she should only produce boys, which is what women were valued for at this time. In fact, Queen Elizabeth's father, Henry VIII, divorced his first wife and beheaded his second because neither of them could produce a male heir for him. So telling a woman that she can and should produce boys is actually a compliment, or arguably so.

Early in the play, as Macbeth wisely wavers on the wisdom of slaying Duncan, Lady Macbeth uses tough language to steel him to the task. She convinces her husband from her brave and ruthless talk that has a warriors' heart. She seems indeed unsexed, as she has asked the dark spirits to make her. From what Macbeth can see, she has none of the softer virtues traditionally assigned to the female gender. These traits would be associated with mothering and nurturing, such as compassion, mercy, and gentleness.

Macbeth imagines Lady Macbeth would raise tough warrior sons with the traditionally masculine traits that Duncan has rewarded Macbeth for having, such as valor in battle. He probably also means she would not raise daughters who would meet with gendered social expectations, such as exhibiting kindness and docility. He is saying that she is essentially a male spirit within a female body. At this point, he seems impressed and a little stunned with her.

Of course, all of this will come to an ironic end when Lady Macbeth works out not to be as tough as she thinks she is.

In act 1, scene 7, Lady Macbeth attempts to persuade her husband into murdering King Duncan against his will. Macbeth is reluctant and tells his wife that they cannot go through with their wicked plan. Lady Macbeth responds by calling her husband a coward and saying that she would have "dashed the brains out" of their baby while nursing it if she knew Macbeth was such a timid man. She then tells Macbeth her plan to assassinate King Duncan and he responds by saying,

Bring forth men children only, For thy undaunted mettle should compose Nothing but males (Shakespeare, 1.7.72-73).

Macbeth's comment indicates that he recognizes his wife's masculine nature and fearless spirit. Essentially, Macbeth believes that his wife is so fearless, bold, and masculine that she should only give birth to male children. At this point in the play, Lady Macbeth takes the initiative, insults her husband, and devises a plan to assassinate the king.

Macbeth says this in Act I, Scene 7, after Lady Macbeth chastises him for his misgivings about murdering Duncan. He tells her:

Bring forth men-children only,
For thy undaunted mettle should compose
Nothing but males.

What he means, more or less, is that Lady Macbeth's spirit is characterized more by what he (and Shakespeare's audiences) associates with masculine virtues. These include courage, ruthlessness, and violence. Indeed, just after hearing of the witches' prophecy from her husband, Lady Macbeth resolves to "unsex" herself so she can become ruthless and pitiless in order to push her husband, who she views as lacking in courage and overly possessed with the "milk of human kindness," to murder Duncan. This would seem unnatural to Shakespeare's audiences, and is an example of what the witches said in the first scene: "What's fair is foul; what's foul is fair." In this passage, Macbeth says hise wife's traits should be passed on to sons, not daughters.