【入门问讳】〖rù mén wèn huì〗[learn about its taboos on going to a friend's house]

入乡拜访随习俗，

门阶进前探清楚。

问知老辈名禁忌，

讳避交谈纰漏出。

古代去拜访人，先问清楚他父祖的名，以便谈话时避讳。也指问清楚有什么忌讳。In ancient times go pay a visit person, first asked names of their fathers and ancestors, so that could avoid talking taboo. Also refers to ask clear have what taboo.