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.
Learn the next connected idiom: 信誓旦旦 (xìn shì dàn dàn) → 旦夕之间 (dàn xī zhī jiān)
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