bosom friend 密友；知己

【例句】
Since she was Hermia's bosom friend, the two of them told her their plot.
因为她是赫米亚的心腹密友，他俩把计划告诉了她。

If you drink with a bosom friend, a thousand cups are too few. If you argue with a man, half a sentence is too much.
酒逢知己千杯少，话不投机半句多。