信誓旦旦

From the Classic of Poetry, poem 58 (《詩經·衞風·氓》):

總角之宴，言笑晏晏。
信誓旦旦，不思其反。 [Pre-Classical Chinese, trad.]

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总角之宴，言笑晏晏。
信誓旦旦，不思其反。 [Pre-Classical Chinese, simp.]

From: The Classic of Poetry, c. 11^th – 7^th centuries BCE, translated based on James Legge's version

Zǒngjiǎo zhī yàn, yánxiào yànyàn.
Xìnshì dàndàn, bù sī qí fǎn. [Pinyin]

In the pleasant time of my girlhood, with my hair simply gathered in a knot,
Harmoniously we talked and laughed.
Clearly were we sworn to good faith,
And I did not think the engagement would be broken.

信誓旦旦

1. pledging in all sincerity; vowing solemnly