Forgive me if I tell you PiakuchuKiller but I think you should review the translation of this chapter.
Mum means mother in any context you put it.
Unless the MC meant that he wanted to forbid his maid from telling the truth to his mother (something very difficult since the mother of the MC hasn't even appeared yet) it doesn't make sense.
I do not know Korean, neither Chinese nor Japanese, but I understand a little basic English as I have studied it since I was a child and the term mum is unforgettable.
Then if you want to explain its meaning to me again, so be it, I'm always ready to learn but really check the translation of this chapter. :/