勤勤懇懇

First attested in c. 93 BCE, in Sima Qian's letter to Ren An:

意氣勤勤懇懇，若望僕不相師，而用流俗人之言。 [Classical Chinese, trad.]

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意气勤勤恳恳，若望仆不相师，而用流俗人之言。 [Classical Chinese, simp.]

From: c. 93 B.C.E., 司馬遷 (Sima Qian),《報任少卿書》

Yìqì qínqínkěnkěn, ruò wàng pú bù xiāng shī, ér yòng liúsú rén zhī yán. [Pinyin]

You expressed yourself with heartfelt concern, as though you were reproaching me for not following your guidance and instead adopting the words of the vulgar.

Note that as quoted above, this word originally meant sincere and earnest. The first quotation in which the word means the same as the usage nowadays is cited below.

勤勤懇懇

1. diligent and conscientious

   • 明旦不寐，勤勤懇懇者，非秦之為與？夫不勤勤，則前人不當；不懇懇，則覺德不愷。 [Classical Chinese, trad.]
     明旦不寐，勤勤恳恳者，非秦之为与？夫不勤勤，则前人不当；不恳恳，则觉德不恺。 [Classical Chinese, simp.]

     From: c. 10 CE, 揚雄 (Yang Xiong), 劇秦美新

     Míngdàn bù mèi, qínqínkěnkěn zhě, fēi Qín zhī wèi yú? Fú bù qínqín, zé qiánrén bùdāng; bù kěnkěn, zé jiào dé bù kǎi. [Pinyin]

     (please add an English translation of this usage example)