最熟悉的陌生人最陌生是熟悉人
[zuì shú xī de mò shēng rén zuì mò shēng shì shú xī rén]
This expresses feelings of estrangement from someone who once was close. Despite being familiar to one another at one point, the connection has become distant or unrecognizable. Often it refers to old acquaintances or ex-partners whose relationships may seem unfamiliar even though they know each other well.