以小人之心，度君子之腹

Its form is based on this declaration by Yan Mo (閻沒) and Ru Kuan (女寬) to Wei Shu (魏舒), to subtly advise him not to take bribes (in Zuozhuan, Zhao 28):

• 願以小人之腹為君子之心，屬厭而已。 [Classical Chinese, trad.]

  ---

  愿以小人之腹为君子之心，属厌而已。 [Classical Chinese, simp.]

  From: Commentary of Zuo, c. 4^th century BCE

  Yuàn yǐ xiǎorén zhī fù wèi jūnzǐ zhī xīn, shǔ yàn ér yǐ. [Pinyin]

  We wished that the bellies of the lowly could become the heart of the noble man, and that he would stop at what was sufficient.

Its meaning is based on this rhetorical question by Ran Qiu (冉求), who alluded to his - an inferior's - own inability to gauge his superiors' minds, to subtly urge Shusun Zhouchou (叔孫州仇) and Meng Yizi (孟懿子) to fight (in Zuozhuan, Ai 11):

• 君子有遠慮，小人何知？ [Classical Chinese, trad.]

  ---

  君子有远虑，小人何知？ [Classical Chinese, simp.]

  From: Commentary of Zuo, c. 4^th century BCE, translation from Zuozhuan: Commentary on the "Spring and Autumn Annals" (2017), by Stephen Durrant, Wai-yee Li and David Schaberg

  Jūnzǐ yǒu yuǎnlǜ, xiǎorén hé zhī? [Pinyin]

  The noble men have far-reaching considerations, but what can a petty man know?

The combination of form (with slight differences) and meaning is attested in a saying attributed to Yu Ai (庾敳) (in A New Account of the Tales of the World):

• 可謂以小人之慮，度君子之心。 [Literary Chinese, trad.]

  ---

  可谓以小人之虑，度君子之心。 [Literary Chinese, simp.]

  From: Liu Yiqing (editor), A New Account of the Tales of the World, 5^th century CE

  Kěwèi yǐ xiǎorén zhī lǜ, dù jūnzǐ zhī xīn. [Pinyin]

  It can be called measuring a gentleman's heart with a petty man's scheming.

以小人之心，度君子之腹

1. to be cynical; to be skeptical of the integrity, sincerity, or motives of others

   • 誰知顏俊以小人之心，度君子之腹？此際便是仇人相見，分外眼睜。 [Written Vernacular Chinese, trad.]
     谁知颜俊以小人之心，度君子之腹？此际便是仇人相见，分外眼睁。 [Written Vernacular Chinese, simp.]

     From: Feng Menglong, Stories to Awaken the World, 1627 CE

     Shéi zhī Yán Jùn yǐ xiǎorén zhī xīn, duó jūnzǐ zhī fù? Cǐ jì biànshì chóurén xiàngjiàn, fēnwài yǎnzhēng. [Pinyin]

     Who would've known that Yan Jun would use his own petty measure to gauge the heart of the superior man [Qian Qing]? This occasion was precisely "foes meeting each other with eyes glaring intensely".

• → Vietnamese: lấy dạ tiểu nhân đo lòng quân tử (calque)