lit. family shames must not be spread abroad (idiom); fig. don't wash your dirty linen in public
Definitions (粵典–words.hk)
語句
指屋企（或者引申到學校、公司、組織等）入面嘅醜事，唔應該俾外人知道，應該私下處理 bad things about one's family (or school, company, organisation, etc.) should not be aired to outsiders, and should be settled within the circle