What is the origin of the idiom "milk of human kindness" and when to use it?

The origin of the idiom "milk of human kindness" can be traced back to William Shakespeare's play "Macbeth." It appears in Act 1, Scene 5, where Lady Macbeth is contemplating her husband's character and his ambition. She expresses concern that he may not be ruthless enough to seize power and suggests that he should be more deceitful and less compassionate. In this context, she speaks of "the milk of human kindness" as an attribute that might hinder Macbeth's ruthless ambition. It is often employed in contexts where individuals want to emphasize the caring and empathetic qualities of a person.