胳膊肘往外拐。(Gēbozhǒu wǎngwài guǎi.)Somebody helps another person, not the one who is closer to himself.

Explanation

按照常理，胳膊肘只能向里弯曲，而“胳膊肘往外拐”的字面意思是胳膊肘往外弯曲，比喻帮助别人（外人），不帮助自己人（内部的人），进而损害了和自己关系更亲密的人的利益。
It is a common sense that elbows can only turn in. The literal meaning of “胳膊肘往外拐” is that the elbow stretches outward. It metaphorically means somebody helps another person, not the one who is closer to himself that does harm to the benefit of the closer one.

Words:

利益lìyì:benefit;

弟弟dìdi brother.

Sentences

1、Jiǎ: Nǐ zěnme néng tóngyì tā de shuōfǎ? Nǐ gēnběn jiùshì gēbozhǒu wǎngwài guǎi.
甲：你怎么能同意他的说法？你根本就是胳膊肘往外拐。
A: Why do you agree with him? I’m your brother, but you help others.

Yǐ: Nǐ shì wǒ qīn dìdi, kě tā shuōde quèshí yǒulǐ.
乙：你是我亲弟弟，可他说的确实有理。
B: You are my brother. However, what he said was reasonable.

2、Jiǎ: Nǐ huì bāng wǒ ba?
甲：你会帮我吧？
A: Will you help me?

Yǐ: Fàngxīn ba, wǒ búhuì gēbozhǒu wǎngwài guǎi de.
乙：放心吧，我不会胳膊肘往外拐的。
B: Don’t worry. I won’t help others to do harm to you.

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