A very close and inseparable friendship, like that between a mortar and a pestle.
他们从小一起长大，是名副其实的臼杵之交，无论遇到什么困难都互相扶持。
They grew up together and are truly inseparable friends, supporting each other through all difficulties.
Next Idiom
A very close and inseparable friendship, like that between a mortar and a pestle.