铁tiě面miàn无wú私sī

Literal: iron face, no personal interest
Figurative: Strictly impartial and incorruptible: applying the rules to everyone alike, unmoved by pleas, favors or friendships.

Where does 铁面无私 come from?

There is no story behind the full idiom, but its first half has a well-known precedent. The 《宋史》, in its biography of Zhao Bian, records that this Northern Song censor impeached the powerful without flinching, and that people in the capital called him "the iron-faced censor." An iron face does not soften or blush when pleaded with or pressured; 无私 adds "with no self-interest." The full four-character form already appears in 《红楼梦》. Today the figure most associated with the idiom is Judge Bao of opera and television, and it is used to praise anyone who applies the rules to everyone alike.

Source (出处): 《宋史·赵抃传》

How is 铁面无私 used? Two examples

小xiǎo区qū的de保bǎo安ān铁tiě面miàn无wú私sī，没méi有yǒu门mén禁jìn卡kǎ，连lián物wù业yè经jīng理lǐ都dōu不bù让ràng进jìn。
The guard at our complex plays no favorites: without a key card, he won't even let the property manager in.
这zhè位wèi法fǎ官guān铁tiě面miàn无wú私sī，不bù管guǎn被bèi告gào是shì什shén么me身shēn份fèn都dōu一yī视shì同tóng仁rén。
This judge is rigorously impartial; whoever the defendant happens to be, they get exactly the same treatment.

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