Daily Chinese Idiom Guessing Game
To pledge or promise with utmost sincerity and earnestness.
信誓表示诚意的誓言；旦旦诚恳的样子。誓言说得真实可信。
《诗经·卫风·氓》言笑晏晏，信誓旦旦。”
He vowed to love her forever with all his heart, his words as earnest as a sacred oath.