na-adjective, noun
Describes something physically or structurally robust, unyielding, or well-built. Also used figuratively for strong resolve or a firm stance.
この城は堅固な造りで、敵の攻撃にも耐えた。
This castle is of solid construction and withstood enemy attacks.
彼の決意は堅固で、
His resolve is firm and cannot be shaken by anyone.
Compound of 堅 (hard, firm) and 固 (hard, solid). The exact historical derivation is uncertain, but the meaning follows from the kanji components.