'A shield' signifies defense which is trusted against evils and falsities. In respect to the Lord, it signifies defense, and in respect to people, confidence in the Lord's protection. This is because it was a protection for the breast, and 'the breast' signifies good and truth, good because of the heart in it, and truth because of the lungs. In an opposite sense, 'shield' denotes evils and falsities which are used to wage combat, and is used as a defense in Jeremiah 46:3-4 among other passages.

(Reference: Arcana Coelestia 1788)