What does Lady Macbeth mean by saying "Come to my woman's breasts...And take my milk for gall?"
This line is from Lady Macbeth's chilling speech in Act I, Scene 5. She has just read a letter from her husband in which he describes his encounter with the witches, who had prophesied that he will become King of Scotland. She resolves at that moment to steel her husband to commit the act that she sees as necessary to his ascent to the throne--the murder of Duncan. So she calls upon the "spirits that tend on mortal thoughts" to "unsex her" and allow her to become filled with cruelty. So filling her "woman's breasts with gall" [i.e., bile] is a reference to that. Shakespeare associates womanhood with gentleness and a nurturing spirit, symbolized most potently by the breasts, which provide milk and nurture life. Lady Macbeth must be the antithesis of the nurturing mother figure in order to carry out her designs for her husband.
At the same time, this phrase alludes to Lady Macbeth's fear that her husband may be too "full of the milk of human kindness," a trait which of course would have been associated with femininity, to go through with the murder. Lady Macbeth will suppress her feminine characteristics in order to drive her husband to meet what she views as his destiny. Shakespeare, with this speech, wants to emphasize not just the wickedness of Lady Macbeth, but the extent to which she embodies the subversion of the natural order of things that accompanies the murder of the legitimate king and the crowning of the usurper Macbeth. This theme, first expressed by the witches, who pronounce "what's fair is foul, what's foul is fair," runs throughout the play.