During the Spring and Autumn period, by the Qi River in the state of Wei, lived a woman abandoned by her husband. In her youth, she was innocent and beautiful. A cunning man took a liking to her, approached under the pretense of trading silk, and proposed with sweet words, swearing, "I love you with all my heart and will never change my mind—I promise to grow old with you."
The naive and kind-hearted woman took his words at face value and agreed to marry him after the autumn harvest.
When autumn arrived, the deceitful man loaded her and her belongings onto his cart, taking her home as his wife. After marriage, the woman loved her husband without complaint, regardless of his poverty, and worked diligently at household chores. But as years passed and her beauty faded, her husband began to mistreat her cruelly, eventually abandoning her entirely. Stricken with pain like needles piercing her heart, she finally felt deep resentment toward her faithless husband and resolved to sever all ties with him completely.
Later, the woman's tragic experience was recorded in the narrative poem "Mang" (The Commoner), written in the form of a first-person confession, serving as an indictment against the injustices of the old society.
The original poem's sixth stanza describes awakening from pain, resentment toward a faithless husband, and the resolve to break ties completely. The lines "He swore solemn oaths, never thinking they'd be broken" mean the man's love vows were once sincere, yet unexpectedly he changed his heart and broke his promise.
Later, people turned "Xin Shi Dan Dan" into an idiom, used to describe extremely sincere and earnest vows.
Source: *Book of Songs*, "Odes of Wei"
Meaning of the Idiom: Later, the Chinese idiom "信誓旦旦" came to describe extremely sincere and earnest vows.