When he starts talking about politics, I just keep my mouth shut.当他开始谈论政治时，我就沉默不语。
2
:to not talk about something (such as a secret)守口如瓶
She told me to keep my mouth shut about the news. [=she told me not to tell anyone about the news]她要我对此事守口如瓶。
I never tell him anything important because he doesn't know how to keep his mouth shut. [=he tells other people what he has been told]我从不告诉他任何重要的事情，因为他不知道如何保守秘密。