bosom friend 密友；知己。人们通常用这个习语来表示自己非常亲密的朋友。当你想用英语表示知己，就可以用这个短语来形容。

【例句】
But she was often sad and never had a bosom friend.
但她郁郁寡欢，连个知己都没有。

A bosom friend is one to whom you could tell all your secrets.
亲密的朋友就是你能将秘密告诉他的人。