八拜之交 (bā bài zhī jiāo, sworn brotherhood)
It is an old saying referring to sworn brothers or sisters with different surnames. "八拜 (bā bài, to kowtow for eight times)" was a courtesy for old family friends to call on the elders.
布衣之交 (bù yī zhī jiāo, friends in days of simple life)
It refers to the relations among the plain folks. "布衣 (bù yī, cloth gown)" were the clothes worn by the folks.
患难之交 (huàn nàn zhī jiāo, friends in adversity)
It means the friends who have been through hard times together.
金石之交 (jīn shí zhī jiāo, close and intimate friendship)
It refers to the strong and solid friendship like metal and stone.
君子之交 (jūn zǐ zhī jiāo, friendship between gentlemen)
It's used to refer to the ordinary but morally oriented friendship. It's a literary quotation from a saying in Zhuangzi: 君子之交淡如水 (jūn zǐ zhī jiāo dàn rú shuǐ, The friendship between gentlemen appears indifferent but is pure like water, or, a hedge between keeps friendship green)
莫逆之交 (mò nì zhī jiāo, friends with complete mutual understanding)
It refers to close friends who share a common temperament and interest. "逆" means "opposite" while "莫逆" indicates "consistent" or "no difference".
贫贱之交 (pín jiàn zhī jiāo, a friend in earlier days when one was poor)
It means friendship made in poor times.
忘年之交 (wàng nián zhī jiāo, friendship between old and young people)
It refers to friends who share with virtue in spite of the difference of age.
忘形之交 (wàng xíng zhī jiāo, spiritual friendship between a noble and a commoner)
It means friends with true hearts and without formality.
刎颈之交 (wěn jǐng zhī jiāo, a friendship to the death)
It refers to bosom friends who are willing to die for one another and never turn their back on each other.
再世之交 (zài shì zhī jiāo, a friendship of the second generation)
It means a person who has made friends with the other and his/her father.
竹马之交 (zhú mǎ zhī jiāo, friendship in youth)
It can be said "竹马之好 (zhú mǎ zhī hǎo)" which refers to the friends made in one's youth. "竹马" is the bamboos that kids put under their crotch and take as horse, which becomes a word to mean the childhood.
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