taru-adjective, adverb taking the 'to' particle
firmly; resolutely; absolutely
A literary or formal adverb/adjective describing an unwavering, determined attitude or action. Often used with と (e.g., 断断固として) to modify verbs.
彼は断断固としてその申し出を断った。
He firmly refused the offer.
政府は断断固たる態度で交
The government approached the negotiations with a resolute attitude.
断固 is a more common and less emphatic synonym meaning 'firm' or 'resolute'. 断断固 is a reduplicated, more intense form, often used in formal or literary contexts.
Reduplication of 断固 (だんこ, 'firm, resolute'), with the second character repeated for emphasis. The exact historical derivation is uncertain, but it follows a pattern of emphatic reduplication found in classical Chinese and Japanese literary styles.