The Vietnamese word "hội kín" translates to "underground society" or "secret society" in English. It refers to a group of people who come together for a specific purpose that is not known to the public. These societies often have their own rules, rituals, and sometimes even a hidden agenda.
While "hội kín" primarily refers to secret societies, it can also imply any group that meets privately to discuss sensitive issues, such as a book club that doesn't share its discussions publicly.