hard, firm
鞏 centers on the quality of being hard and firm, like something solid and unyielding.
鞏 combines 巩 (a component associated with holding or securing) with 革 (leather), suggesting the idea of something made firm or hard like secured leather.
Leather (革) that is secured (巩) becomes hard and firm. Picture a piece of leather stretched tight and held in place, making it solid.
For キョウ, imagine a firm leather shield that makes a loud 'kyo' sound when struck: kyo -> キョウ, and the shield stays hard and unyielding.