People should mind their own business. This expression apparently first appeared in print in an O. Henry story in the early 1900s.
curiosity killed the cat
It's best to mind one's own business. For example, Don't ask about his divorcecuriosity killed the cat. This cautionary expression sounds like the moral of some fable or folktale, but any such origin for it has been lost. The first recorded use was in O. Henry's Schools and Schools (1909).