知己用英文怎么说？

From text book, the "bosom friend " is the right one, but people never say that way. The popular word for that meaning is buddy.
look: Hi buddy!
He is my buddy.

简明汉英词典

知己
[zhījǐ]
bosom friend confidant

汉英词典

知己
[zhī jǐ]
(情谊深切的) intimate；understanding
和他很知己
be on intimate terms with him
(情谊深切的人) bosom [intimate] friend
用法
知己，心腹 [xīn fù]:见“心腹”。

Google 网络词典
知己
confidant
【摘要】 11．confidant 知己 confident 有信心的 12．country 国家 contrary 相反的 13．cow 牛 crow 乌鸦 crown 王冠 clown 小丑 14．dessert 甜食 desert 沙漠 ...

bosom friend
【摘要】 bosom friend 知己 bosom 胸部 bosomy 丰满的 boson 玻色子 bosquet 丛林 boss 老板 boss-eyed 斜眼的 bossdom 势力范围 bossed 有浮雕的 bosseyed 斜视的 ...

Soul Mate
【摘要】 05 Soul Mate 知己 06 Distant Love 两端 07 So Good So Good 08 Big City Big City 09 Have You Heard Lately? 如果你也听说 10 Summer Wave 夏天的浪花 ...

confidante