A similar expression is used in King Lear (milky gentleness, (1.4.340). We have to believe from Lady Macbeth's description that Macbeth is capable of great compassion and affection. For much more please see the annotations at the bottom of the page for Macbeth 1.5.
How to cite this article:
Mabillard, Amanda. Macbeth Glossary. Shakespeare Online. 20 Aug. 2000. (date when you accessed the information) < http://www.shakespeare-online.com/plays/macbeth/macbethglossary/macbeth1_1/macbethglos_milkkindness.html >.