taru-adjective, adverb taking the 'to' particle
Describes a strong, unwavering attitude, belief, or determination. Often used in formal or written language with たる or とした.
彼は確固たる信念を持って行動した。
He acted with firm conviction.
確固とした態度で交渉に臨ん
She approached the negotiations with a resolute attitude.
断固 also means firm or resolute, but often implies a decisive, uncompromising stance in action, while 確固 emphasizes an inner, unshakeable state of mind or belief.
Compound of 確 (certain, firm) and 固 (hard, solid). The reading かっこ is a kan'on reading for both characters.