The Webster Dictionary’s definition of “bosom friend” is “intimate or confidential friend.”
The Oxford English Dictionary’s definition is “A very close or intimate friend.”
The Cambridge English Dictionary’s definition is “a friend that you like a lot and have a very close relationship with.”
The Urban Dictionary defines the term as “An intimate friend; a really kindred spirit to whom you can confide your inmost soul. Very hard to find.”
I like the last one.
An English-Chinese Dictionary would usually translate it as “知心朋友” (zhī xīn péng yǒu), which back translates literally as “a friend who knows your heart.”
Just as in English, where there is more than one way to describe a bosom friend, e.g. “kindred spirit”, or “soul mate”, etc., in Chinese there are also several terms that express the same meaning as “知心朋友”.
“知心朋友” is an expression used in spoken (vernacular) Chinese. In classical Chinese, the Chinese used in ancient times for written works, expressions were abbreviated to their bare minimum, because paper hadn’t been invented, written works had to be carved on bamboo slips. That means a lot of carving and a lot of bamboo, plus the space to store these works. The classical Chinese phrase for “知心朋友” would become “知友” (zhī yǒu), omitting the two middle characters; “知交” （zhī jiāo）, replacing “友” (friend) with “交” (association, relationship); or “至交” (zhì jiāo), replacing “知” (know) with “至” (extreme, utmost), note the same spelling of “知” and “至”, but different intonation).
There is another, I think, more elegant expression in classical Chinese for describing a special friendship — “知音” （zhī yīn）, a friend appreciative of one’s music.
And there’s a story behind it. This happened during the Spring and Autumn Period (770-476 B.C.E.). Yú Bóyá (俞伯牙), a qín (ancient Chinese zither) player, was playing his instrument. Zhōng Zǐqī (鍾子期), a woodcutter returning from work, heard Boya playing, and remarked that Boya’s tune was “lofty as the towering mountains”. Then Boya changed his tune, and Ziqi remarked that his music was “magnificent as the flowing waters.” Whatever Boya played, Ziqi would grasp the former’s feelings. They became “知音” （zhī yīn） (a friend appreciative of one’s music). When Ziqi died, Boya felt that no one else could understand his music, so he broke his qin and cut the strings, and never played again. Thus the term zhi yin has been carried forward till this day.
Billy’s Comments: Harry is my cousin-in-law , married to my cousin Yihua Li. He is one of the most interesting persons to talk to – full of passion and has deep knowledge in both Eastern and Western Cultures.