屎坑公賣草紙 [－－－卖－纸]

Jyutping si2 haang1 gung1 maai6 cou2 zi2
Pinyin shǐ kēng gōng mài cǎo zhǐ

Definitions (粵典–words.hk)
  1. 語句
    歇後語​：​問心​。​以前​啲​人​去​完廁​所​係​唔​係​用​廁紙​而​係​用​草紙​，​而​呢​啲​草紙​通常​唔​係​免費​，​所以​「​屎​坑公​」​，​即​係​舊​時​嘅​廁​所​清​潔​工​，​會響​廁所門口​疊​草紙​隔籬​擺個​箱​等​人入​錢​落​去​。​草紙​同錢箱​都​係​無人​看管​，​入​唔​入​錢​都​全​憑​啲​人​嘅​自​覺​，​所以​就​引申​到​咁​嘅​意思​。
    to appeal to one's conscience; to need to be honest, often used as an instruction or warning; literally: toilet attendant selling toilet paper

屎坑公賣草紙 [－－－卖－纸]

Jyutping si2 haang1 gung1 maai6 cou2 zi2
Pinyin shǐ kēng gōng mài cǎo3zhi

Definitions (CC-CANTO)
  1. to be honest with oneself; to just trust someone to be honest and do the right thing even if others are not looking [colloquial]