Lady Macbeth:Macbeth Act 1, scene 5, 15–18
Glamis thou art, and Cawdor, and shalt be
What thou art promis'd. Yet do I fear thy nature,
It is too full o' th' milk of human kindness
To catch the nearest way.
To Lady Macbeth, the "milk of human kindness" is distasteful stuff—no self-respecting man has any use for it. Therefore, when we use the phrase to approve someone's compassion, we reverse the original sentiment.
Lady Macbeth is ambitious, and fears that her milky husband lacks the mettle to grab the Scottish crown in the most expeditious manner. "The nearest way," as she sees it, is to murder King Duncan. She hatches this plot—which had independently occurred to Macbeth as well—when he writes home that three witches have prophesied that he would be created "thane" (lord) of Cawdor, and later would ascend the throne. The first half of the prophecy has already come true, and Lady Macbeth is in a hurry to make sure the second half comes true too.
As fluids go, Lady Macbeth is more inclined to murderous blood than nurturing milk. Later, goading the hesitant Macbeth, she insists that, if she had sworn to do it, she wouldn't have hesitated to take her own baby "while it was smiling in my face" and to "Have pluck'd my nipple from his boneless gums,/ And dash'd the brains out." A charming woman.
Speakers: Lady Macbeth